Delbertine I didnt tackle that issue yet with my conversion. However since I live in Propane country do we have many Vehicles that are powered by propane. They have underbody tanks that are filled at the gas station. Newer tanks have an automatic shut off so it stops the pump as soon as your tank is at 80% (legal filling limit) older tanks have a little bleeder valve which you open when you fill your tank, as soon as you see propane escaping from that valve (misty vapor) the tank is at 80% and you have to manualy shut off the pump. These tanks work great if you have access to a propane fueling station which is no problem up here but could be quite the task to find one in other areas. If your stuck There is a adaptor hose which will screw on to your fill valve and to a regular portable propane tank. Connect the hose turn the bottle upside down and open the valve, propane will flow into tank.
I've also seen RV's with this kind of tank. So dont know if that is what you got.
How long will a fill last? All depends on your appliances and how often will you use them. A 20lb bottle holds 5gallons of propane, 1 gal equals ca. 91'000 btu's so in a 20lb you should have 455000 btu's. With this you could do a rough calculation, figure out how many BTU's your appliances use, guess how many minutes or hours you use each every day. Calculate the BTU's used and if you want to calculate cost too remember that all your appliances are not 100% efficient, depending on age and model it could be anywhere from 60% to 95% (95% very expensive appliances, most likely not found in a Skoolie conversion.)
So theoreticly you could run a 35'000btuh furnace on a 20lb bottle for 13 hours. (of course ir real life you could deduct a coupple hours)
I'm not sure if a 20lb / 5gal. bottle actualy holds 5 gal. or if it is filled to 80% of 5 gal. Maybe someone else knows this.