It's a big tent then with hard walls to take the boiler!
i am assuming caravan/motorhome or static/mobilehome. further down forum under vehicles.
My thoughts are a leak somewhere at a joint or rubber hose in perished state letting in air as well as seeping gas hence the fire going out after 20mins.
Best get a GAS SAFE engineer test the whole system. Any repair work is covered under GAS SAFETY as in household and you can't work on it without it being tested after, and you can't get engineers who are willing to test work they haven't done.
BB, January 2011
