Keep in mind, you're not using all these propane appliances at once. The fridge takes very little propane but would be on all the time unless you're plugged in. The stove also hardly takes anything and you're not cooking constantly so it's overall very little use. The old ones are heating 6 gallons of water whether or not you're using it and if you don't turn it off when the water is heated, will cycle on and off heating water for no reason, turn it off when it's heated. The new ones have tankless systems, thereby reducing propane usage quite a bit.
That leaves your furnace. While the newer ones are supposed to be more efficient, they're operating mostly only in the winter and for short bursts, but they are a relatively large consumer of both propane and battery power.
The generator is the other propane hog on the older models. On the newer models, the need for the generator is reduced to only when you need the air conditioner. Everything else (except the fridge) will run off the whole house inverter.
So, when you add it all up, there's not a whole lot of propane being used in vast quantities unless in the winter if you leave the furnace on.
The size of the tank also depends on the model and floorplan you're looking at. I believe the tank sizes are different for the cutaways, versus MB vans versus the Promaster.
Hope that helps some.