are they of any help in increasing mileage? and, do they shorten the life of the engine? Any you would recommend for a one-ton GMC 2004 truck that pulls a fifth wheel rv?
As I understand it Duramax is a diesel engine not an add on. There could be hundreds of possible add oon. What specifically do you have in mind.
For increasing mileage, the best thing you can do is regularly scheduled maintenance.
You could replace the chip in the ECM with a multimode chip. They come with a switch that you mount under the dash. You toggle the mode to economy, middle range power, or stump puller. They might not be worth the cost in fuel you save. Google them and see what you find.
You will not increase your mpg no matter what you add on. Not as long as you’re towing a 5th wheeler.