2005 Chevy 2500HD Diesel Towing
 

Recently, My truck seems to have lost some power while towing my boat. I used to be able to do 70-80 MPH no problem. Now, I am only able to go 60-65 MPH without the truck losing speed as if I am going up hill and wanting to kick down into a lower gear. Driving in trailer mode doesn't make a difference. I am not getting any type of check engine light.

I am wondering where I should start looking. My instincts are telling me to maybe start with an engine tune-up with Air filter cleaning and maybe a fuel system cleanup? My fuel filter is currently at 50% life.

I just wanted to get an opinion on if I should focus on the motor or transmission first. Any feedback is appreciative.

Thanks,

How many miles on it?
A diagnostic tool is going to make your job MUCH easier, if you don't want to be shooting blind and wasting your money.

Coming up on 300k.

I have a pocket code reader but given I am not getting any codes, not giving me much.

Any diagnostic tools you can recommend?

Thanks,

It will take a fairly high end tool to diagnose the transmission in case that's giving out.

Unfortunately, I am not a diesel guy so I can't help you much

With a gas engine, you could do things like a cylinder compression test, fuel injector test, measure exhaust back-pressure, measure fuel pressure.
It might be better to pay a professional to diagnose the issue for you.