I have a 2003 Silverado 1500 HD 6.0. The truck has 89,000 miles an Airaid air filter and flow master exhaust all was installed back in 2004.

The truck seems to be sluggish. I notice it during daily driving but worse when I'm towing a travel trailer that weighs 9100 lbs. I live in Florida so its flat road towing. Today I weighed the truck and trailer loaded with wife, kids, gear and food. The total weight was 15,030 lbs.
What can I do to get more power? Is she just old and I should step up to a 2500? One time back in October I did get the infamous Reduced Power, while I was towing. I pulled over checked everything found nothing wrong, had lunch and it went away. I've towed twice since then and it has never come back.

 

Thats a 9500# camper, a little to much for a 1500 hd and a 6.0 Mine tows my 7800# easy

 

The truck is supposed to tow 10,200. I thought being under by 1,000 would be good.

 

Weight is one thing, wind resistance is what is hurting you; you could probably pull a 12000# boat ok. Most travel trailers have around 64 sq ft of front surface (wind resistance) tall 5th wheelers have even more.
That truck souldn't be sluggish empty though, mine hauls butt stock.. Maybe go by an auto parts place and have them check it out with a scanner. 6.0 engines are know to run over 300,000 miles.