This sounds like the thermocouple has gone bad, almost bad or out of position not getting warm enough by the pilot light . The bulb as you are calling it is the thermocouple. When it warms and gets hot enough it allows the gas valve to open and keeps it open. Since you are doing this with your grill lighter or other heat source you are over heating it, to make the thermocouple strong enough to open the gas valve.
Those are the basics of the thermocouple job. If you end up replacing it, need to make sure you get the same one Atwood spec'd out for it. I do not know the exact one used in your older camper.
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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