If the previous owner says that it was working but it has sat for a while, you could just have the old bug issue. The pests can and will crawl up into the furnace. They love the odor put in propane. If they get anywhere near the gas orifice or tube, (maybe a small web) it will prevent the furnace from igniting. It cost me almost 200 bucks once to have mine removed on a 92 class A and it was the most expensive insect I have ever purchased. Always a good idea to block off the intake and exhaust of the furnace if it is going to be sitting idle for a while. There are other reasons that a furnace may not fire, low batteries not powering the fan well enough, no propane, high or low limit switches, poor spark, etc. if you know there is gas getting there, hear the fan blowing well, and hear the igniter, it is probably the insect issue.
Steve and Sheri with Archie (and Hiro, R.I.P.)
2000 Winnebago 35U, Ford F53/6.8l V10
F150 Ford and Vintage Aspencade.