As for the propane, go by the outside gauge, it is in the tank, and while maybe not 100 percent, it is the better gauge. Others have mentioned the black tank solution, maybe.
If you search in here, some have recommended 5 gallons of water, then two bags of ice, and go for a ride. The ice will help scrub the walls of the tank, knocking anything loose.
2012 Ford F250 ext. Cab 4x4
2002 Jeep Wrangler Sahara
2008 Yamaha V-Star 650 Classic
2008 Work and Play 18LT