I just had the same issue on my 1976 31 Sovereign
. I have owned this trailer for 20 years and never had the problem. But all of a sudden I had weak gas at the stove, the furnace cycled a lot and the air it produced was lukewarm at best. The pilot for the WH only stayed on if nothing else was drawing gas.
My first thought was to replaced the 10 year old regulator. Even though my trailer lives inside I thought that it may have gone bad.
I was wrong, and the symptoms remained. The copper was all fine, and the appliances had worked find just a few weeks earlier!
The only thing it could be was the 24" rubber line between the regulator and the copper. It was original, but who knew? I know from experience that rubber brake lines swell over time on the inside, and eventually become so occluded that the brakes work very little and can even lock on because of the trapped pressure.
I cut the old line off of the copper ( the flared fitting was so old it was seized) and cut the hose into sections. At the flared end things were fine, but by the time I got to the crimped ( regulator end) of the hose, the opening had swollen down to 1/8". I ordered a new hose from Vintage Trailer, and when it arrived it looked like a pipeline by comparison. New flared ends on the copper, a quick installation, and I can now bake bread, make popcorn, run the WH, the fridge, the catalytic heater, and the propane lantern AND the furnace and all runs well!