What would cause the furnace to use more propane?
There are several answers. MOST of them are "Maintenance" issues
Some there might be an obstruction in the flame/exhaust path. this would reduce the ability of the furnace to heat. It might also cause an OVERTEMP shutdown or a fire so that's a serious one.
A better guess is air/fuel mixture (Air gate) it is adjustable.. now I know how to adjust it but.. I'm not comfortable giving instructions READ THE FINE MANUAL or better yet an even better manual or ask someone who knows. It's easy you need basically a screwdriver (Phillips on mine) fingers eyes and ears.
Last (This is not the furnace) Air leaks and/or poor insulation on the "House" (RV in this case)... A drafty house burns more propane.