First truck - 2006 VNL670 - mileage question

Hi everyone -

First post here. Thanks for the information that's shared - I find most of it to be helpful.

I just bought a 2006 670 w/D12, 10 spd trans, 3.58 rear end from a guy that I've known for a while now. My uncle is an experienced driver and was looking for something better, so I bought it for him to run. I run the business end and he drives. I don't have a CDL (I'm a tax accountant by day) and admittedly don't know a ton about diesel trucks (although I'm learning every day), but I've rebuilt 15+ gas engines and I used to buy/fix/sell cars for extra cash so I have a decent mechanical aptitude.

My main question is in regards to the fuel mileage that can be reasonably expected from this truck. It has 900k on the engine. We're just getting started, but it looks like the guy I leased on with is giving him some heavy loads, some light loads, some in-between. I understand weight will impact the mileage, but I'm trying to get some real world examples for guys with a truck with similar specs, and what changes you've made to achieve that mileage. Also, how do you typically drive and what weight are you normally hauling? This is just basic dry van although I might consider getting a reefer once I get some of my investment back as the guy has some nice paying reefer loads.

I've been doing a lot of research and plan on getting either an Airdog or FASS system installed, an OPS-1, and maybe switch over to Schaeffers oil. I don't know when the last time the alignment was checked, but I might have an alignment done so I know it's good. I've also read that it might be wise to get a valve adjustment done. The truck has new tires within 5k miles before I purchased it, so I'm kind of stuck with them for now - whether it's keeping duals and getting something with low rolling resistance or switching to super singles possibly. The truck also had a new trans installed 2 weeks before I bought it and it has a 2-yr warranty, so it doesn't make sense to even consider messing with the gearing in any way. Finally, if I need to get an in-frame done at some point, I know that Volvo has the Guaranteed Overhaul program which would give me a warranty on the motor as well - I'm just not sure what would be the max expected fuel economy to decide whether it's worth the in-frame sooner rather than later.

Ideally, if I could consistently get 6.5 - 7 mpg I'd be happy. I'm also paying my uncle based on percentage of what the truck does, so if I can decrease fuel costs, we both make more money.

Any input would be greatly appreciated.