You need to monitor live data and see that the engine is making it to operating temp, entering closed loop and the oxygen sensor is switching.

Yes timing will affect fuel mileage.

What is the mechanical condition of the engine? Oil consumption, oil pressure, compression, cylinder leak down? Are you expecting to much from a worn down 210 hp 350?

 

How would I go about monitoring live data on a truck like this?

How bad would off timing really hurt it though?

I would expect around 15 on the highway to be honest. Especially with a camper shell on the back smooth and clean. It really isn't that "old" as it was rebuild two years ago. (Do not know the quality of who built it, but the block was cracked in the original)

 

Blue Point Ya 3140 OBD2 1 Scanner w Complete OBD1 Kit | eBay

I gave one of these to my dad 3 years ago. It works really well. Never used it on OBD1 but it should work.

 

You have a basket case right now, need to address the issues you have first.. Timing can make a big deal in power and fuel eco. You need to get all that in check, then figure out your MPG's

 

I can agree with that. The thing is, I want to do an engine swap on this truck. (Yes, it's my project truck and I want to do something a bit less common)

With that in mind, I have to find a happy medium between spending money to fix little problems and saving for the big stuff.

Right now I can't drive my truck on base, because it stays in first gear the entire time. I can, however, drive it to our searches because they are usually an hour or more away and I'll be in 4th gear 90% of the time.