HELP!! Furnace won't work

I had this same thing happen on a Suburban furnace. It turned out to be the gas valve. The furnace would cycle on, but since no gas was getting to the burner it would not light, so the fan would run for a while and then shut off. Check first to see if the thermostat is bad by bypassing it. If that's not the problem, then the gas valve may be it. I would not recommend tackling this one, as the valve is imbedded in the guts of the furnace, which needs to be removed from the RV. Special wrench was required on mine, which I didn't have anyway. The valve may not be a stock item anyway. Good luck