It could be possible that the motor isn't running as fast as it should be, but still moving enough air to satisfy the sail switch. The switch is there only to make sure there is enough air flow to purge any fumes from the fire box before ignition, and to make sure the fire box stays cool. Sounds weird, but if there was no air flow the burner would get so hot it would start things on fire. Should be able to feel a noticable amount of air coming from the floor vents when the furnace is running.
Another possibility is the propane tanks. Tanks need to be purged of air and moisture when new, and few if any are actually done (most dealers sell the trailer with full propane tanks, but won't spend the money to have them purged). Then they need to be purged every so many years to eliminate moisture/condensation from the tank. hard to explain but basically it lowers the BTU's and the propane doesn't burn as hot. Check by lighting a burner on the stove. The flame should be a pretty blue color with none or very little yellow on the very tip of the flame. If it's mostly yellow with hardly any blue than a purge should be done. This was explained to me by my local propane guy. My flame was pretty much all yellow. After being done I did notice a difference in how the furnace, water heater, and stove run. The furnace air seemed hotter, and my gas use went down a little. And the cost at the time was only $3 a tank. Not sure if this may be the fix, but cheap enough to try.