I am running TOA with a weekly group, and enjoying it.  But I am frustrated that there is no terrain key for the map of Chult.  (This is not unique to DDB-the print book is the same way).  It looks to me as though the terrain color/textures are consistent across multiple maps, so the map from LMOP is somewhat helpful.  Is there a master key for terrain for these maps somewhere?

I think the main folded map in the back of the books has a terrain key I believe.

Nope; there is a key, but it only lists things like cities, ruins, mines, etc.  No key for the different terrain types.

Part of the frustration for me is that there are terrain types in the wilderness (hills?, grassland?) for which there are not encounter tables).  Part of it it is not being sure which terrain type is "wasteland" (which is listed on the encounter tables.)

If you look to the southern portion of the map of Chult there is an area listed as Valley of Lost Honor. The terrain type that the valley resides in is "wasteland"

Yeah, that's what I was guessing. I'll just have to punt with the hills, etc. in terms of random encounters.  Not a big problem.  Mostly I wondered if there was  kind of "universal key" for the terrain used in these maps, since it seems consistent across adventures.

I think that they made the ToA random encounter chart to supersed the DMG one for the adventure to express the uniqueness of Chult's ecology.

I've been wondering the same thing. When the party crosses hexes that contain hilly terrain (at least that's what it looks like to me), which random encounter table is meant to be used? Just go with whatever the PCs have been traversing prior to crossing the hilly hex (e.g. "Jungle, no undead" or "Mountain" or whatever)?

I had to deal with this last session. I decided to go with the encounter (from the various terrains) that made the most sense in context. This time, they were near the mountain range that includes  Hrackhammer, and Hrackhammer is their current destination. Since the encounter roll I made came up as fire newts in one of the terrains, that's what I ran with. 

Note:  I "roll" Random encounters  ahead of time, using the random number generator in excel, as part of a larger spreadsheet with weather and other notes. One page gives me 10 days of weather and encounters. When I prep for a session, I use the private notes in the campaign section here to list out all the encounter options depending on terrain and sometimes to determine numbers of creatures, etc. ahead of time.