Ignition doesn't appear to be the problem, so, the two major causes would be a) a spider web in the gas tube (spiders for some reason love the smell of LPG) or b) a bad thermocouple that is sensing an "over temp" condition and killing the gas flow prematurely.
I would start with verifying that nothing is living in the gas tube constricting the gas flow...that would also cause the system to abort the heating process. The gas tube (flame tube) should be open and clear of debris and when lit, should burn blue with very little yellow/orange flame along its length.
A "normal" length burn for a 6 gallon tank should be in the 5-10 minute range depending on ambient air temperature...
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)