Sleeping six is not typically a probllem, each end sleeps 2 and the dinette will sleep 2. Fridge, because of the low height closed up, a tall fridge is not possible, they are usually dorm type, either 2 or 3 way. (110/LP or 12/110/AC) Furnaces. Usually about 12k btu, and with canvas sides, not much R value. Good to about 40 deg. Maybe less depending on sleeping bags.
Batteries, they usually only have 1, but all you are running is the lights unless using furnace, then battery usage counts alot. Often you can add another batt. Solar, get what you can, but be aware to use the solar right you need system to control charge. Lots of good solar info on web.