According to an '85 Ford Data book, a base (no options) F250HD 4x4 regular cab has a curb weight of 4358# with the std. 5.8L V8 and T-18 trans. A base truck has next to nothing else and does not even include a second fuel tank. The same data book has the weights of all the options, so get out your calculators:
4358 base truck, plus:
676 diesel option w/4-speed (includes dual 83 AH batteries)
653 diesel option w/automatic (includes dual 83 AH batteries)
186 aux fuel tank (part of 4x4 diesel package but not included in package weight)
96 fuel tank skid plates (part of 4x4 diesel package but not included in package weight)
100 frame mounted spare & carrier (part of 4x4 diesel package but not included in package weight)
56 argent step bumper (part of 4x4 diesel package but not included in package weight)
46 Chrome step bumper
18 XL package
29 XLT Lariat Package
102 AC w/auto trans
75 AC w/manual trans
53 stabilizer bars
39 aux springs
52 suspension package B
5 radio
All that said, my '86 automatic w A/C weighs about 6200# with full tanks but little else but bigger tires (38 gallons of fuel weighs 272#), so the OP's weight sounds in the ballpark to me.