A few months ago I had a similar problem with my furnace. It got to the point where the fan would come on, but no fire. I did several things to eliminate the problem, but still didn't know or understand enough about these furnaces to solve the issue. I had good gas flow and pressure. Beyond that, I was pretty much clueless.
Alas! My old friend YouTube came knocking at my door, and I found this YouTube video:
The location of my sail switch was right on the front, easy to remove and I tried cleaning it. Well, that did not work. I ended up going to my local RV dealership that sold Keystone RV's and they had the switch. There was a slight assembly required, but I got it. I attached the wires, screwed it back on and ... whaloa! It worked. The furnace has been running flawless ever since.
In fact, before I changed the switch, my furnace would always attempt to fire. It would finally fire on the 3rd attempt. It was like this from the day I purchased the camper. I just thought it was the nature of the breast. After replacing the sale switch, it now fires on the first attempt, even after the propane tanks have been turned off for a while.