I've had a few GM trucks with 305's and 350's with Edelbrock carbs,I got about 13-15 mpg--well tuned carb,fresh tune up,stock 235/15 tires..you could squeak out 17 mph back before we had oxogenated fuels and ethanol in thegas--you lose 3-5 mpg with alcohol in the gas in my experience,especially on older carbed V8 engines..
I'd say ditch the big tires,keep the lightweight rims,put stock tires on it,and drive it gently,if you dont see any improvement I'd have the carb looked at..I used to use either a 500 CFM or a 600 CFM carb,anything bigger is a waste of gas on a small block Chevy..make sure the choke opens fully and quickly--I opted to use a hand choke,because I had flooding and hard hot restarts with the "automatic" ones,they tend to close before the engine needs choking,and wastes gas and fouls plugs..many days above 60 degrees,I didn't use the choke period,or only for the first mile after leaving the yard..when I swapped a 500 CFM Edelbrock with manual choke on my 305 in place of a 600 CFM with an automatic choke,I gained 5 mpg,went from 12 to 17,in a '79 C-10 2wd I had,with 3.73 gears..
I also was able to advance the timing about 4 degrees above stock specs without excessive pinging,that proabaly accounted for some of the mileage increase,also adding a adapter/spacer under the carb to bolt it to a factory quadrajet manifold gave me better driveability and slightly better fuel economy around town..(actually a well tuned Q-jet will beat practically any other 4 BBl carb for fuel mileage,due to the dinky primary barrels--the Edelbrocks are almost all the same size on all 4 barrels,but they are still pretty fuel efficient for a performance carb)...best carb I had for mileage was a Holley "Economaster" 450 CFM 4 BBl..had one on a '74 454 and I got 13-16 mpg with it no matter what--loaded,empty,it never changed!..
Bottom line is your truck weighs 5000 lbs probably,and might be low geared,and have the wind resistance of a plow,so you'll never be getting much over 15 mpg,but you should be able to go 300 miles on a 20 gallon tank I'd think,your right foot has a lot more affect than you think--the vacuum gauge is a good suggestion,I used one in many of my trucks--by watching it you'll notice you can often back off the throttle a lot,yet still maintain the same speed..keep that vaccum higher than 15 inches,and you'll be getting roughly the same amount of mpg..
My 81 G-10 van with a 73 307 V8 and a Holley Economaster 2 bbl carb has gotten 18 mpg on long trips,but its a real dog as far as acceleration--top speed is about 75-80 mph,and its slow getting there--but with 2.73 gears,I can get it up to 72 mph in SECOND!..third just kinda bogs it down,hill climbing is not its favorite thing really..its no fun to drive to VT,NH,or ME,its slow!..but its good on gas!..