I searched for this, but couldn't find anything so sorry if its a repost.
My 04 SCCL has slowly over the past year decreased in HP.
Towing my empty 18" car trailer nor requires most, if not all (on hwy overpasses) power to make it.
I used to be able to tow my Jeep and trailer no problem. I wouldn't dare take it over any hills right now. 30K miles, no tune ups yet other than oil changes. Anyone else experience this? Any recommendations?

 

Not familiar with what SCCL is but try disconnecting the battery for five minutes and they reconnecting it to reset the computer.

 

sccl - Super Crew Cab Lariat

I didn't even think of the basics, thanks for the reminder.
I'll give that a shot and see what happens.

 

You mention that you've not done anything other than regular oil changes, right? If so, then you should replace your air filter & fuel filter. With 30,000 miles, both of those could easily be dirty enough to effect performance in a negative way. Both are cheap and easy to do. Also, what brand of gas are you using? Have you switch brands over the last 30,000 miles? Could be a result of poor quality fuel.