Sounds like a fun project---if it goes right!
Not an AC/heat pump guy here, so keep that in mind but I do feel okay with electronics in general so might be able to help some.
First thought would be to steer you away from a different board, but I might question how you went about choosing the new thermostat as it seems the thing which is new and not wanting to play with the existing. Not to say you are wrong but more to try tracing back how you got that one? Possible the new has a different layout for th plug pins and switching a few wires is needed?
Part of my concern with wiring 12v direct is that it needs to go through the load shedder or you lose that function.
Have you come across this drawing for your specific year, make, model of RV?
Note that page Two is for RV with heat pumps.
http://www.winnebagoind.com/diagram/...ire_138913.pdf
I might go with this plan if I had a thermostat with a wiring chart to ID connections on it. I might go with one wire at a time, matching the wire use on the RV with the wire ID on the new thermostat. But that is where getting the exact correct thermostat is critical as heat pumps do throw an extra wrinkle in things.