I have an 02 2500HD Duramax and when i bought it, the best i could get out of it was 13. I put a cold air intake on it and a superchip programmer on it and now i get about 19 out of it without a trailer. The superchip is a pretty mild tune, it only adds 75HP but it makes a huge difference in throttle response, not too sure if it would help you when pulling a trailer.
2002 Silverado 2500HD Duramax/Allison ECSB
1996 Silverado 2500 ECSB
1995 Reg Cab 3500 w/ Dump Bed
2006 Ferris 4500Z 61" CAT Diesel
2005 Exmark 48" Turf Tracer
2005 John Deere 647 Quik Track 48"