I have a 1989 Bigfoot
17 and the original 12K BTU Atwood furnace that was in it when I bought it was dead. Since I drag this thing between Alaska and California, often in sub-zero temps, I wrestled with this same issue for awhile before I solved it.
I didn't want to buy a new furnace in Alaska as they are incredibly expensive here. But I had to get home last fall
in freezing temps so I opted for a Mr. Heater Big Buddy portable propane furnace to at least get me home.
While it worked, I later realized how much better a built-in furnace is. The heat from the portable is hard to control in a small space. It seems like it was either too hot or too cold. Plus the heat just goes straight up and collects on the ceiling before it starts warming up the areas at bed height.
When I got back to California I bought a new Atwood 7920-II 18K BTU unit to replace the 12K BTU one that was dead. I bought it from Chuck at Fiberglass Travel Trailers RV
and I've never looked back.
This January I drove back to Alaska and even in temps well below freezing the furnace kept the trailer plenty warm with little effort and kept the heat evenly distributed. It was night and day from the portable.
The lesson learned is that you should "try before you buy" if possible as you may never know what you are missing if you try one solution that you think is "good enough". It depends on where you intend to travel and in what kind of climates so there is no single "right answer". But you should make an attempt to try different solutions like a portable or wall mount heater and a built-in propane heater to better judge for yourself before you make this decision. Even if you have to borrow someone else's RV or trailer to try it out, I highly recommend this as the decision i the end is very personal and subjective.