95 fleetwood furnace

Yes, it could be a dirty sail switch and by tapping on the gas valve it may move enough to close the contacts.
Another thing is to look at the gas valve. Most have two coils that both need to operate the gas valve. Locate the four wires going to the valve. Should be two wires connected together going to ground and two wires connected together going to the circuit board. disconnect a set of wires and check the coils for continuity. Each coil should be very close to being the same resistance.
The furnace may need a good cleaning. Blowing out with compressed air will do wonders for getting dust and other debris.

my camper is a travel trailer, 35 ft Fleetwood prowler, with the Atwood 8500 furnace.

If tapping the gas valve causes it to come on, that suggests a bad valve.

BUT, start with the basics. If the furnace fan runs, but the furnace does not light (gas valve open AND igniter click), the SAIL SWITCH may not be closing. Causes of this: low voltage (measure at the furnace) causing fan to turn too slowly. Restriction in air return and/or vents and ducts causing reduced air flow. Or, the sail switch may be bad (least likely).

Quick check-- start engine and hold at high idle so voltage is up. Any different symptoms?