Under normal conditions, the furnace combustion exhaust is completely separate from the hot air supply (two different chambers inside the furnace) and the combustion is discharged to the outside of the trailer.
Unless there is a defect in the furnace somewhere, even having a duct or ducts that are not completely sealed, should have no effect on making it smell (especially of propane) since the heated air all comes from inside the trailer and is recirculated.
I have seen instances of where someone had stored chemicals in the basement compartment and a non-sealed duct sucked the chemical smell up into the trailer.
2-1/2 tanks of propane through a furnace is an awful lot.
Under normal circumstances, that is MANY, MANY hours of burn time!
Many folks can go a whole season using just one tank so if you have used 2-1/2 tanks just trying to 'burn the furnace in' there is something wrong.
Have it checked by someone qualified in furnace repair.
2016 Flagstaff 8529IKBS Diamond Package 5th Wheel
Ford F-Series SCREW 4x4
TST Tire Monitor - Sinemate 3500w Gen.
2016 Nights Camped = 46