Thanks, ***** ***** outside access it makes things difficult. IF the furnace is firing off and shutting down it could be an issue with the LP pressure, electrode, high limit or electrode. With no outside access you are really limited. The first thing I would do in the shop is check the LP pressure with a manometer, the appliances need 11" of water column to work properly. This would be hard for you to do. Go to the LP tank and shut the valve. Slowly open it back up, sometimes if the valve is opened to fast the regulator can lock up and you might not get the proper pressure....
Next would be to test the circuit board, normally the circuit board is in back of the furnace, if you do not have a side panel inside to get to the circuit board the furnace will have to be pulled. The electrode senses the presence of the flame and will send a signal to the circuit board to shut down if it does not sense the flame. Many Rv shops have a special board tester and can tell if the board is bad. If you can get to it I would have it tested. IF the board or electrode is not the problem then it could be the high limit shutting down the flame, the high limit is inside of the furnace.
I see this is a 2015 since you have no access panel then I would let a shop take a look, you should have a warranty on this and it should be covered. IF you start taking to many items apart then you could void that warranty, I have seen it before.
Hope this helps, IF you have more questions or concerns please feel free to contact me... Robert