If you want a furnace check out Propex, I looked into the Truma's but there's no US distributors for service/warranty like Propex has.. they seem like nice units otherwise.. If you have a VarioHeat I'd love to hear more about em, not much info from actual users.. Ive got a smaller propex in my uninsulated westfallia and it does great.
Camco Wave heaters are well worth the cost over a buddy if thats your style (no electric, burning air you breathe), they burn pretty much all the propane they consume and dont stink up the place.. they also work at high altitude and can be recess mounted if you'd like.. Buddy stinks, is made of plastic (my handles been broken for ages), wont stay running or even start at altitude, and consumes alot of LP.. but it does put out alot of heat for cheap (if you ignore the cost of feeding it 1# bottles nightly)
My current (much larger than yours) travel trailer build i'm doing both, the Propex for thermostat controll (no overheating on mild nights) and daytime use (no hot elements for kids and pets to encounter) and the Camco for backup heat source and base heat on sub freezing nights where the 9500btu Propex is not enough, without incurring any additional electrical penalty w/the additional heat output.