I have a 66 Tradewind with the original furnace. It works fine, but hey it is 48 years old and has two problems- first I don't trust it; there may be a leak in the heat exchanger. Second, the furnace fan is kinda noisy and uses too much battery. This is a problem as I mostly boondock.
I just ordered a new furnace today. It is a 12,000 btu/hr Atwood unit with spark ignition and a very efficient fan motor (1.8 amps). This is smaller than the original furnace which was 20,000 btu/hr, but I think it will be fine. I don't usually camp if it is much below freezing. The photo below came from the Technical Installation Manual.
I would never install an ac on top of a 66 Safari. Totally overkill and it really ruins the looks of a very attractive Airstream. I carry my small 5,000 btu/hr AC unit in the bottom of a cabinet until I need it then install it in a side window. I designed a mounting bracket for it and it takes less than 10 minutes to install. 90% of the time we camp, we never need to install it. Here are some photos, and you can read a thread on it at the following location:
You may also want to look at a thread called "dan's 66 tradewind improvements". Feel free to pm me if you have any questions. I will help you all I can.
Good luck, Dan