Switch from 30 lb. to 20 lb.propane tanks?

Round these parts, at least, refilling a 20 lb cylinder is not generally charged by either weight or gallons, but rather by a flat fee. The actual filling is generally done by weight (i.e. put the cylinder on a scale and fill until it's 20 lbs over the tare weight, or until the OPD valve won't let any more in). I don't know how they handle other sizes of cylinders.

For the non-removable LP tank in my RV, thus far I've been charged by the gallon.