I have the same issue with my extended rear overhand class-C pulling a 20' enclosed. The ULTIMATE (aka. expensive) solution is to use a 'Trailer Toad' (http://www.trailertoad.com/). However I've found if I go up one additional link on my load leveling bars it definitely helps but does not keep it from happening 100% of the time.

Also, due to the extended rear overhang, make sure the rear frame extensions are re-enforced to support the trailer tongue weight. I've seen more than one extended rear class-c break at that point due to a heavy trailer. I re-enforced mine by welding in a 3' section of steel c-channel at the extension joint on both sides.

HTH,
Dan