[jsb5717]

Thanks for the update.

First, it's most likely not the sail switch. If it were then there would be no attempt at all to try to light. The fan would blow then shut off.

With the gas valve apparently working (smell of gas present) and no sounds of ignition, then my guess, like yours, would be the main board and/or ignitor. I'm not sure about the older furnace but on ours the ignitor is part of the main board. Do you have the original paperwork for the furnace? Ours came with a helpful parts list with check marks on the parts relevant to our furnace. That might tell you about the main board and ignitor. If you can locate the model number for your furnace you might also find what you need online.

Given the added difficulty of having to remove the furnace to service it you might also spend some time watching YouTube videos on RV furnaces. There are some helpful ones out there that can also help to identify the issue by the symptoms it's displaying.