If you can't get it started, apply some pressure on the wrench and rap on the wrench several times with a hammer. The vibrations will often loosen it up. If you have someone else to hammer while you hold the wrench, it will be easier.
If all else fails, you can rent an impact wrench.
2011 Silverado 2500HD Duramax, 4x4, crew cab, long bed
Palomino Puma 253-FBS, 27' 5th wheel
1994 19' Class B on Chevy chassis