Could my fuel milage be excessive

[THEBRAIN]

Could my fuel milage be excessive
05 pleasureway lexor rd chevy 3500.w/ 116K miles on clock, one carry on bag & laptop bag an sleeping bag.small tool box
No gear or water in tanks kindof lite by traveling standards.

This calculation isn’t exact. I had a tad less than ½ tank which is 15 gallons.
Cruised 155 miles mostly 2 lane black top county road.so that’s 10mpg.
This mpg seams off by 5mpg?
Maybe time for a complete tune up?
I have a new air filter an fuel flilter yet to install (will next nice weather day)

The engine idels an runs so smooth I didn’t think she needs new spark plugs.
Remember I don’t have much history of my new van.I believe GM says to leave iridium
SPs in for 100K mile is this true or false. You would think SPs in that long would be welded to heads.

Please advice on tune up parts like spark plugs I normalley use NGKTR55 on my
LS1 car, an my L33 V8 trailblazor.

What SPs are you Guys using w/ the GM 6 litre?
I'm planning on a decent SP socket this https://www.amazon.com/gp/product/B0...JRE9G6AY&psc=1

or is 6" the wrong lenght, was planing to remove front wheels an do SPs though wheel well.

thanks for advice.

I'll probably remove the roof AC, I think it's 85LBs.

photo yes my rear wheel needs clean an repaint untill I get nicer rims

edit my rear tire preasure was 75 and fronts where 55 both cold.
I have read preferred preasure is 80 rear 60 front. 5LBs couldn't lower the mpg down to 10 or could it?