They will NOT fill your propane tanks to the 'full' level.
When you pull in to have your RV propane tank 'filled' they are required by law to only fill it to 80%.
Took me awhile to figure that out too. Seeing as after you have it filled most of us don't look at the gauge before we leave. They fill, we pay and then drive off. Then when we check it later we wonder why we've gone thru so much propane. :R
On my tank my gauge registers 75% when I have it so-called filled.
Gotta admit that when I have an extended stay at a CG that has propane on site, some of them will fill it full for you. Albiet I am sure they would get in trouble if someone reported them. I seek out getting propane at CG's because my tank is so small and that extra 1/4 tank means a lot if headed for a dry camp site.