The usual suspects are in order of likely failure on an Atwood/Dometic are 1. Sail switch 2. Board 3. Limit switch 4. Electrode/Flame sensor. If it's firing at all it's not the sail switch, the sail switch is either open or closed, there's no in between. The limit switch on an RV furnace is really a high limit switch, if it's stuck open the furnace won't ignite, otherwise it's purpose is to cut ignition if the chamber temp exceeds 190°F. The board is the furnace brain, so if it's bad nothing will function correctly. The electrode fires the furnace and the flame sensor signals the board that flame is present.
The sail sw., limit sw. and electrode are the cheapest fixes and I'd replace those first, if still no success replace the board. At the worst you'll have spare parts