Gas furnaces are accessed from the outside of the RV. From the pics I could find of your model, it looks like yours is behind the cover plate that has two vent looking holes with a chromed plate covering that section of the furnace. Looks like it's right above the water heater access panel.
Depends on model and build by Jayco, but generally, first remove the screws from the chrome plated vent tubes, pull them out, then remove the black metal cover. That will give you access to the sail switch, and a view of the hot box and other parts. Then there are usually a couple screws holding the furnace in place. Gas off, disconnect gas supply, thermostat and power wire bundle, and slide the furnace out of the cubby hole. There may be a fuse on the control board.
There is a lot of goop sealing various cover parts and takes some tugging and prying to get those covers off.
Here's a picture of what they look like after the covers are off and here's a link to my ad free blog article that may help...
Furnace: