Well, you need the gas valve under the trailer open FULLY (valve handle inline with the pipes) *and* the gas valve on the tank open. If your heater is one of the autostart, pilotless models, then you may need to cycle it, waiting ten minutes between tries, before it will fire. I lit off mine for the first time this winter today, and it took three tries before it would light. I watched in the little window each time, and it was obvious when the gas got to it - it lit right off.
I then let it cycle off and back on, which it did several hours later. I had it set for 80!
Tomorrow I'll dust out the cat heater and give it a try, too.
I bought this trailer in Michigan, and they were SERIOUS about their heat. It has the furnace, the cat heater, and the heater strip in the old Armstrong A/C.
Hmm... I just reread. I don't think you could try lighting one of the pilotless models with a match. If it has a standing pilot, I'd try some other things, too. One is to determine if the fan is blowing enough to trip the sail switch, which has to happen before it will turn on the gas.
If that's working, then the gas valve would be the next thing to check. Also, some of the heaters of the 70's had a service bulletin on them about an air "pipe" or hose that would crack and provide a CO hazard. A search of the forum should find more info about that.
If you're not comfortable messing around with propane appliances, be safe instead of sorry and take it to a professional. (Who will, of course, want to sell you a new heater.)
Good luck with this.
1975 Argosy 28 "Argosy"
1979 Excella 500 31 "Betsy"
1992 Lincoln Mk 7 LSC
2003 Dodge 2500 Cummins "TowHog"
"Lucy Loosehair" the cat - Airstream mascot
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