Kw w900l

My uncle is ritiring & asked if I want to buy his 03 kw w900l midroof sleeper. He bought it back in 08 with 425k mi. It now has 685k. He drove from Illinois to Texas. Seasonal only. He took very good care of it. Maintained it very well. Inside & out is in great condition.
It has the C15 mbn engine
10 speed trans
475 hp
3.70 rear ratio
11R24.5 rubber. 5k mi on tires

He hasn't done any major repairs to it. The engine has not been opened up. Any part he has bought has been from kenworth or CAT. He's had the same mechanic since he bought the truck.
He's asking for $25k. Which I believe is a fair price for the condition it's in. & comparing it to other w900s on truckpaper.com
I will be hauling light dry van loads. 15k max cargo weight. 300 mi radius from Chicago il. (il,in,mi,wi,ia) so mostly flat land

Ive done a lot of research on this website about mbn engines. I know they have their ups and downs.
He says he gets around 5mpg. He drives for economy. I know there are a couple things you can do to help the power & mpg. He said its stock so he hasn't done anything to it.
I guess I am just looking for some clearification. Some tips on what to keep an eye on? What to keep my ears open for? Which way is best to run the truck? (rpm, mph) Will these specs work for me? What to change?

I will be going to look at the truck soon so I should get more info on it. & I will for sure take it somewhere or have someone with knowledge on these trucks & engines look at it before making a decision.
If I go thru with this I will be interested in doing what I can to get better mpg.
Sorry for the long post. I appreciate any help or advice or tips or contact information I can get. Thank you very much.

 

@DUNE-T i know right. That's my biggest concern. But I've read a lot about these other guys doing some tune ups to theirs & getting closer to 6 mpg

 

Yeah I did a lot of research on them. I don't think it's worth the headache. Gonna look for something more fuel friendly.

 

I guess I fell in love with the interior as well. Also because the truck is well taken care of. So I thought I would deal with the mpg. But I will rather have more money in my pocket at the end of the week.

 

I actually read a lot of your comments you posted in other threads. Thats where I got most of my research from so thanks!