Welcome to the board. Just a little fyi, if that front Dana 60 is a kingpin axle, it alone is worth what you paid for the truck. Even if it has ball joints, it's still worth what you paid. The truckload of spare parts just sweetens the deal. One good thing about that body style is there is good aftermarket support for it.
As for you getting crappy fuel mileage, the turbo 400 is partly to blame. It's a very good transmission, but its lack of overdrive is part of what's killing your mileage. That, and I would venture to guess your truck has 4.10 gears. I can't remember if that came with lockout front hubs, but if it didn't, they will help your mileage a little bit also.
The fuel filler neck on those trucks has always been a pain. I had an '86 dually that was the same way. I ended up making a bigger fill tube for it with a straighter shot to the tank. Look behind the fuel door where the tube is and it will make sense.
Post some pics up when you get a chance.
JB

 

it has lockout hubs IF its a 60, oh and IF it is a 60 its a 3500 not a 2500.....

 

welcome.

 

Welcome to Diesel Bombers

 

Welcome

 

It IS a Dana 60, I was told it has the 60 up front becuase it has the hi serria package which "sets it up" for a camper, it had a slide it camper it the back before the guy I bought it from had it. I have a few dana 44's at my house, its way bigger than they are.