This is the first I've ever seen about parts not being available for an SF-42, which is not even that old as things in the RV industry go. The most common failures are on the blower, the control board, loose wires, the sail switch, and the high-limit switch. All of those parts are readily available.
Can you describe exactly what happens or doesn't happen when the thermostat calls for heat. In normal operation, the thermostat calls for heat, there is a relay overhead above the air conditioner air return that clicks, then the furnace blower starts and runs for about 25 seconds. There are up to 3 ignition attempts. If successful, hot air will start flowing within a few seconds.
What does yours do?