Diesel Bombers - New Guy Here

I got my rig '83 GMC 2500 for $800 last December. It's got a 6.2/t400/np208/ dana 60/70.
247,000 KM on the odometer. It got the Hi-serria package and it was originally sold in Canada
It had new radiator, starter, alt, injectors, glow plugs, rotors, caliper, pads, shoes, wheel cylinder, brake hardware, drums, exhaust, shocks, tires, 2 batteries and cables, e-brake cables, all installed about 3 months before I bought it.
New in Boxes, case of each: fuel/water seprators: fuels filters: oil filters, 15 glow plugs, blower motor, A/C (evap, compressor, condensor, hoses, orfice),4 gallons oil, 2 extra new batteries, brake pads, 1 ton tie rods and steering linkage, alternator, fuel tank selctory switch, wiper motor, heater switch, 12 gans of antigel,

It's ugly but runs good, I don't think its that slow like some people have said on the web. It gets crappy milage, under 10 (any clue why?). Its also hard to put fuel in it because of the angle of the fuel filler hose that goes down to the tank, anybody have this problem before? The engine is supposed to have around 70k on it. I got the old heads with the truck and the old injector pump which was working when the old engine was removed.

I towed a 18' enclosed car trailer with it(empty). Only thing I pulled with it, the truck didn't think much of it.