Might want to crank the heat towards the end... I would think it would work the same way people put chicken on a hot grill at the end.
I don't cook a log of pigs, maybe 2 a year... but if you want crispy skin you need render out a lot of that fat that sits under the skin. I'm not sure if you're getting it all out at those times/temps. I typically end up with about 1 hour per every 10lbs hog cooking at around 325-350.
I don't do them for contests... just personal stuff, and for friends...
Also, "gas oil drum" does that mean its propane? Depending on the configuration, burning propane for that long in a closed cooker is going to generate moisture. You may have too humid an environment...