Some general thoughts:
1). Bigger than you need.
2). Plus expansion tank.
Have not said, “Buy this one”. Work down from here, just not too far.
— The
propane system is (literally) the answer to, “what is a camper” (any type).
— But it’s the
water system which makes camping this manner the pleasure that it is.
Electrical System is what gets gold-plated and yet IS NOT necessary EXCEPT to power furnace fan.
Go over the whole of the water system. Consider adding filters if you haven’t (built-in). Make sure drain valves are “great”, etc. Survey entire system.
Water is what destroys RV campers. Roof separation and plumbing leaks or breaks. With the exception of behind-wall electrical junctions,
nothing else is as destructive (what totals an RV).
Water pump makes or breaks all else in that system, IMO. (Duty-Cycle considerations). Wanting a constant-flow water heater? Outdoor shower to wash things not just people? Wash dishes while kids take shower? Adding a washer/dryer or dishwasher (which, ironically, uses less water than by hand-washing). Transferring water from camper to another, exterior storage tank? (Do you carry a water pump mounted for portable use?).
The more people, the greater the demand. The unusual circumstances of fate also play in.
Water
storage capacity is the limiting factor to,
how many days without resupply for X-people in Y-circumstances.
A propane system has portable lightweight bottles to be filled.
The RV itself
has to be moved for water re-supply. Consider how you’d do it otherwise; filter, treat, exchange the camper fresh water tank contents. Etc
I’m trying to say it’s not hard to imagine that which would overtax a minimal-spec pump.
.