I assume it is a Duo Therm 65810-002. There is not much that can go wrong with these. The thermocouple is a good place to start. Be sure it is in the pilot light
flame path. Maybe look for a voltage on the leads. It will be in the millivolt range. If this is not the problem, then look for blockages in the propane
lines. Spiders are fond of building webs in propane
lines. Worst case scenario is that the gas valve needs to be replaced. This should be done by a qualified propane
tech. The original valve is no longer available, but there is a substitute:
Robertshaw® 710-201 Unitrol Gas Valve 710 Series 7000SLC Right Valve® Equivalent
The original valve is a 41-0640, as shown in this spare parts list:
The Robertshaw valve claims to be the replacement for this valve. The problem is that it has different port locations, so some tube bending will be required.