The bummer has been that I've encountered a couple
> of RV stops that have a minimum charge of $12.

I think this is a good argument to install an "extend-a-stay" after the valve.  I installed one when bus depot had their sale, and love this little gadget.  It allows me to run from an external propane tank very easily.  I have a small 8 pound I think it is, and bring that along on trips as a back up. You can even run from disposable 1 pound bottles in a pinch, with the proper connectors.  It also works the other way if you want to power up an outdoor grill.  I use mine to fuel an eco-temp for hot water that I hang from the rain gutter, but you could fuel a stove or whatever. Recommended!

Troy