On our Adventurer, the furnace gets removed by removing the panel that vent comes through. It's moderately involved. You have to remove some trim to get to the screws that secure the upper part of panel to get access from outside. There is weather sealant that you might be able to use, or not. If you remove the furnace, you need to turn off the gas to the coach before disconnecting the gas line.
*Some* troubleshooting can be done from the coach's inlet air vent. As I recall, you can see the gas solenoid and perhaps the sail switch from there.
The furnace gas valve is operated by that solenoid. It's not a handwheel-operated valve. The valve/solenoid assembly is easy to identify, because it's got two wires and two gas connections (one in, one out). You *should not* jumper the solenoid to turn on the gas unless the fan is running. The fan starts first, and runs a bit to make sure the combustion chamber is clear before the solenoid operates. This is likely intended to prevent a gas explosion.
I have never had our thermostat off the wall (no need), but most of them just make a connection. If that's true of RV thermostats, one could eliminate the thermostat by connecting the wires long enough to see whether the furnace starts. That and the fuse would at least tell you whether you need to get to the furnace at all. Just because it clicks does not mean it's working.
I just went removing and reinstalling our furnace this last year. In our case, it was the control board. We got the Dino one and it works great. Our symptoms were different, though; the fan would start but the gas solenoid would not open and the ignition would not fire.
1976 Southwind 28', '96 Winnie 34WK,
2006 Tiffin Allegro Bus 40QDP