1st Thread, saying Hello and ?'s

[csdineley]

Hello

Been hanging around for a couple months now. I've been around old squarebody's most of my life. My parents brought me home from the hospital in my dad's brand new 79 k20 diesel. Then I learned to drive in his 82 lwb 305/700r4. And in high school I had a 79 G20 van with a 350 Q-jet/ th350. But for the last 15 or so years, I've been doing the sport car thing and then the kid hauling thing. At the end of last year I decided to get another truck. And after 4 months of searching for one that was as stock as possible. In January I found an 87 R20 350tbi/th400 4.10. One owner who just pasted a way and the truck had been sitting in his garage for the last 5 years. It only has 90k on it and is as far as I can tell is bone stock down to the original Gm floor mats.
So I've been going through it and tuning up everything since it had been sitting for a while. New plugs,wires,cap,rotor,oil,filter,trans oil and filter, fuel filter, 2x sending units, 2x fuel pumps,power steering pump and hoses,water pump, radiator,thermostat,both temp senders,belts and O2 sensor. And running every test I can think of compression, timing, voltages, etc,etc,etc. The truck was and still is running great, just thought after sitting so long it could use a good tune up.
Now for my questions. It's only getting 10mpg. Which seems a little low to me. I wasn't hoping for high numbers when I got it. But I thought it should at least get around 13-15mpg. I drive only on back roads doing a steady 55mph with no stops (unless I get stuck behind a school bus but that's rare). And have a round trip to work of 30 miles. This is the same kind of mileage my old van got. I thought the tbi should help mileage. In comparison my dad's 82 got 18mpg on his 120 round trip everyday for 10 years. So am I missing something,is there any thing else I should check? And does this mileage seem right to you guys?
And yes I know it's a 3/4 with a 3 speed and 4.10 gears, none of which helps my mpg's. Also should state tires are 245/75r16

Thanks
P.s Sorry for the long post