Ask your propane supplier, they're the experts. When I was doing my gas fitter training, we spent about 10 minutes on propane tank/cylinder sizing because, as my instructor said, it's only the supplier who really needs to know.
Ayuh,... Agreed,... The tank valves tend to freeze up, under high usage like that too,...Generators use a large amount of gas, so the tank needs to have a lot of surface area for the propane to boil off to vapor. 100 gallons tanks don't have enough surface area, so if you were on the generator on a cold night, you won't have enough gas pressure to run the furnaces also, and maybe not enough to keep running the generator.
A single 250 gallon tank should have enough surface area to handle a 5000 watt generator.For reference information only, I have one 250 gal. above ground tank that does a good job supplying a 80,000 btu furnace at the same time as a 36,000 btu water heater when the outdoor ambient temperatures is -21°F.