The propex is pretty easy for a DIY aftermarket install and will do what you want it to do. I would avoid the newer Atwoods, there are anecdotal reports of reliability issues and the power consumption of these has actually gotten worse in the most recent models. I would also avoid the unvented heaters (Buddy, Wave etc). Condensation is already an issue in these minimally insulated campers in the cold, and dumping all your combustion products into the camper is only going to make it worse.
I am not sure what your budget is, or if you want hot water, but given the choice I would go with the Truma Combi:
Truma Combi Plus Propane Furnace and Water Heater *Extremely low stock remaining of eco plus (120V units). We will not be able to get more of these once we sell out.* FREE UPS insured shipping to US or Canada ($150 Value!) What is the Truma Combi? The Truma Combi is a combination furnace and...
trumaheaters.com
It is efficient (on both electricity and propane) but most importantly it ha variable output so it can run continuously on low, which makes for much more even heat and much quieter than the propex/atwood options. The drawback is that it is pricey and the retail options to purchase are slightly gray market.