Heavy Haul - Fuel Economy and Engine Life

[KWT880]

All,

I have a;

2019 Kenworth T880 with a X15 565/2050,18sp, 4.11 gears, 24.5 tires
2450 hours and 60,000 miles/98,000km
8 axle set up - Tri drive truck with a quad wagon logging truck
Loaded - 140,000lbs/63,500kg Empty - 43,000lbs/19,500kg
Cruise speed - 55-60mph or less depending on the road
Fuel Economy - 3.7mpg avg

I run in extremely mountainous terrain, from 8% on the highway to over 15% off road
All this leaves me with terrible fuel economy but being an O/O I want to try and maximize the fuel economy as best while trying to minimize engine wear. I keep seeing "keep the boost under 15lbs" but with my application, weight and terrain that isn't a reality for me, I would go backwards down a snow covered bush road. I try to keep it as low as possible while maintaining enough speed to keep going forwards. I also try and drive as slow as I can while still making my multiple trips per day during the available mill scale hours and HOS. The truck is still new but what sensors should I be cleaning or checking? What EGTs should I try to stay around? What help, advice, tips can you guys offer me to help me keep more money in my pocket?
I've added some pictures for you guys to check out.

Thank you