IF you want to boondock for as long as possible then you compromise on the things that will otherwise end your trip. Camping at a campground or camping for one night is a lot different than boondocking where there is no water, no hookups, no electricity. Water, then power (unless solar or gen) then propane dictate your stay length. Carry extra or use less.
**also, Toadtoes, your 20 gallon tank might hold 20 gallons but the outlet to the water pump is an inch high, so there could be 1-2 gallons (more if the outlet is higher) left in the tank when you run dry. Then you have to tilt the trailer to get a little more...Then there is that same 1-2 gallons or more of dead air at the top if the air vent is on the side of the tank as many are. so you have 2 gallons of unreachable water at the bottom topped off by 2 gallons of useless air at the top, so, yes, your 20 gallon tank could be as you say...
