I don't have an auto standby gen but quite a few neighbors do. Here they're on natural gas and they do use a lot. If running a large whole house unit, say 15k watts it will run through a lot of propane and that will cost you. Natural gas is cheaper and always available.
As for the gen in your 5th wheel, it could be used if you know what you're doing. 5500 watts will run most everything. No central air but it will keep your furnace running, the lights on and most everything else. We have a 5500 watt gen and run the furnace, well pump, 2 refrigerators, deep freeze, lights, TV, internet. We also use the microwave, toaster oven, coffee pot, vacuum cleaner, etc. Just don't try to use all those other things at the same time. A little common sense, turn off things you don't need and 5500 watts will keep you going.