So I have an interesting one shot coming up in January or February and plan on it being a dope horror story. It is a modern day campaign where the party is a group of friends going on a camping/hiking trip. They arrive, nothing happens. Everything begins with the hike. The day after arriving, they go for a hike and encounter they monsters that will hunt them down. NPCs will be with them and they will be the first to die. Good chance a few party members will go with them too. I need advice, ideas, etc. Thanks!

Hmm, well how you run it depends on you, but for a realistic setting I recommend avoiding most magic heavy classes and non-human races, unless you have a different plan, then feel free to use them.

But regardless, if you are in need on monsters, I have a few ideas.

Ghouls and zombies are great for horror, skeletons are too, but personally I think the nightmarish ghoul who can paralyze people and the rotting zombie that refuses to die are scarier then the animated bones.

You could also use the Scarecrow from the monster manual, it can paralyze people from afar.

Giant spiders also help.

For a horror story, you need to have them encounter the monster but turn tail and run and be able to escape it. In a lot of horror movies they manage this by having the monster be slow but sneaky, or slow but have an alternate way of moving that the protagonists can't see (e.g. air vents). 

Oozes might be your best framework for that, they are slow but high-damage and can climb walls but they don't fit the woods all that well.

Zombies could also work.

Alternatively, you can have a fast monster that feeds on those it kills so each encounter leads to the death of a couple NPCs that then serve to distract the monster so the party can escape.

My monsters are two of my own making that probably closely resemble other monsters cause I didn't check. One is Jawdrop, a pale white humanoid who has dimly glowing eyes and  jaw constantly open, displaying his set of bloodstained teeth. Then we have the Maw, a tall charcoal grey human with a featureless face, with only his mouth remaining. Both are supposed to be the paranoid inducing, is it watching us? type monster. The forest is where they will be for the majority of the campaign. This is one of those situations where a lot of people go in and very few come out. Also throughout the campaign, one or two NPCs go missing. They later find the bodies in... rough condition

I also wanted to see if anyone had ideas for subclasses adapted for modern day. Also I wanted to see if having a fusion of magic and tech would make sense? I think it kinda doesn't but I wanted to get others opinions

Why wouldn't any of the classes / subclasses work for the modern day? Just replace all the crossbows and bows with guns, and change some flavour text and you're basically done.

The biggest thing you will have to combat in a D&D game is the heroic mentality - most players have the mentality of "these foes exist for us to smite them!", and with 90% of character abilities tending toward combat, the game engine reinforces the idea that fighting is the preferred outcome.

Now, with a horror game, fighting typically leads to death. The players should be focussed on survival - avoiding combat, hiding, fleeing, and working out how to escape. This does not mesh perfectly with D&D, but it is stil ldoable.

My first suggestion is to have two "evil" factions. One will be your Maw and Slackjaw monsters, who will represent the unstoppable terror. The other might be something like wolves, or some other dangerous but not unnatural force against which combat can occur. Gnolls may also make a good option.

Second suggestion - make it lower level. The higher the player character level, the more tricks they have and the more hard counters they have to your threats. At level 5 they can start flying using the Fly spell, which may invalidate both of these monsters. They can also start trying to use Hold Monster and other shenanigans which may take the threat off your enemies - unless you make them immune, in which case it is a double edges sword - the higher level a character is, the more you want them to feel powerful, so the bigger impact making them powerless is. It'll be far easier for a party of level 3's to be caught up in a horror story than a party of level 13's. The moster lunges - Banishment! - and it's gone.

Third, but perhaps most important - tell the players it's a horror game, and tell them in no uncertain terms that if they treat it like a hack & slash "enemies are XP" game, they will die very quickly. Player buy-in is a must.

Fourth, give an introductory encounter which has no uncertain terms about the enemy capabilities. Maybe telegraph a high-level paladin NPC who is ripped apart without the enemy missing a beat. If gore is acceptable, perhaps the monster rips off their legs and drags their torso into their lair, so there's a reason for the monster to leave without killing everyone.

Now, onto more specific ideas!

If you're considering an attrition style game, where PCs are picked off one at a time, then you need to make the entire game a puzzle - combat encounters won't cut it, because combat = death. So, have a think about the lore behind your two monsters. The exciting part of the story behind these sort of things is uncovering the lore of the enemy. What does it want, where does it come from? You know that "bingo" moment when Freddy realises Jason is afraid of water? That's the moment you should be shooting for. Was the Maw drowned to become like this? 

For a really tense game, I just came up with an idea for the following:

1: Level 3 characters, expect a horror survival game where combat is not ideal. Don't make a character all about combat or you'll find yourself lacking.
2: Limit the races to those without darkvision.
3: The game is a series of challenges and puzzles to try and escape (consider the Quarry videogame, where the only way out is the cable car)
4: It takes place over one night
5: The monster is attracted to light.

So the party will be trying to solve the puzzles against a time limit (use a real timer) and not use any light. Certain events cause light (getting the generator started, for example) which attracts the monster. You're aiming for that "Event, realisation, reaction" sort of game. You describe the whole scene - "The generator sputters into life and the lights in the room come on. You hear the buzzing in the transformer room next door, and..." and so on - and if they notice the lights they will freak out, and if they don't - Jumpscare!

Make some events cause effects elsewhere - you try to get the phones working and the electricity comes back on, and the shed - where one of the PCs is hiding - lights up. Even better if you make it christmas themed, so the lights on the house are timed to activate at midnight!

For other horror mechanics, consider using a timer, using Suspense Dice (where every time they do something risky, you put a dice in a bowl. When something fails, you roll the dice, and if any are a 6, the monster comes), and tracking the characters HP yourself, so they never know how hurt they are.

Good luck!

Thanks! This will help a bunch. Just hope my players survive (maniacal Laughter)