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Light up a stove top burner for a few minutes to equalize the LP in the system. IF you see any yellow or orange in the flame tips that can be a sign of low LP pressure at the tank regulator. That should be checked and adjusted at least once a year. Then try cycling the furnace at least 5 times. IF it still won't stay on, there is a good chance it may need a new control board. That can be a trouble spot on these with the OEM boards. Be sure that the batteries are fully charged and the tanks full. Try to be plugged into shore power if you can so as to rule out low DC input voltage. IF you need more on this later or if you need help on getting to the board, please get back to me as all follow ups are included in our fee. IF you include the make of your furnace, I can add a copy of the service manual at no extra charge. I will also make the phone chat option available for you in case you feel that would be more helpful here.
And you don't access the furnace from under the coach. It either has a removable outside panel or you pull it out from the inside. That can be tricky sometimes as it requires taking the LP line loose. Please get back to me if you feel that is necessary.
IF you do need a new board, I would go with an aftermarket product made by Dinosaur Electronics as they are far superior to the OEM boards and have an outstanding warranty. The model of the board you want is called a "Fan 50 Plus" model.