I had an interesting situation just last month. I filled one of my two propane tanks and reinstalled on my Designer. I switched to this recently filled tank and no propane would flow. The tank was obviously full. I turned off the valve on the tank, removed the hose and then reinstalled the hose and "slowly" opened the valve. Still no flow. I then removed the hose again with the valve off and took a small screwdriver and opened the overflow on the tank for a couple of seconds. You know the one they use when they are filling the tank. I then put the hose back on and "slowly" opened the valve. Success. I had propane flowing. Apparently purging a couple of seconds out of the overflow worked. I'm not sure why, but it did.
2006 GMC 2500HD CCSB 4x4 Duramax/Allison, Titan 52 gallon fuel tank, Prodigy Controller, B&W Companion Hitch
2010 Jayco Designer 35RLTS, Cummins/Onan RV QG 5500 EVAP, Progressive Industries EMS-PT50C, TST Systems 507 TPMS, RV Flex Armor Roof