Propane weight/gal varies with temperature, but the RVIA uses 4.5 lbs/gal for the official calculations used in RV specs. That means your 86 lb tank holds about 19 gallons of propane per RVIA calculations, but more like 20 lbs using the official 4.24 lbs/gal.
But make sure that 86 lbs was for the weight of propane. The capacity stamped on a horizontal (ASME specification) tank is the water capacity in US gallons, not weight. DOT specification, portable vertical tanks have their capacity stated in pounds of water
. How's that for confusion?