Wind resistance kills the M1009. I pulled my 2000 pound flat bed car hauler last March empty and it felt like it wasn't there for a 60 mile drive. Load 1 old pick up truck bed/converted trailer that stuck up into the air and I couldn't hold more than 52 mph pulling up some hills. No mpg numbers for that trip.
Going to the Spring TX Rally the same month had me pulling the 101 with the top on. 52 mph was all I could do against the wind. 15 mpgs for that trip. Which included several hours of off road crawling too.
Last July, my wife had a break down in our 2000 Suburban. I went to go get her with the M1009 and the flat bed. Winch and hitch troubles had her in the M1009 flat towing me in the Suburaban with the trailer attached to the Suburban. 48 mile drive and as long as she didn't try to drive more than 45 mph, the hills never slowed her down. No numbers for that trip either.
August had me pulling a 5x8 box trailer full of heavy stuff and 4 Scouts 130 miles round trip. 15 mpg with top speed around 65 for most of the drive.
However, since I changed the IP in the M1009 I have discovered comparing one 6.2 to another is like apples and oranges. It all depends on the condition of the pump. I had no problem with 65 mph as my top speed empty before. I had to push to run 70 on anything but flat ground, my mpg was great (20-22) on long trips at 60 mph.
I changed the pump and now keeping below 70 mph is a challenge. I did 140 miles Friday night on some really hilly, twisty roads. Attacked the hills to maintain no less than 69 going up and let it drift up to 73-74 going down the hills. 17.1 for that tank. I am trying to keep it below 65 mph for this next tank. Had to drive 70 miles last night and the thing has so much more power now found myself having to work pretty hard to keep it below 65. It wanted to run.
So, back to the original question. While pulling a big load is possible, it is not recommended due to wheel base, suspension, hitch and braking concerns. Go to a civi K5 forum and they will tell you 8K is a doable load. As long as it is long, low and flat with great surge brakes I guess you could. I wouldn't.
Mileage depends on load, wind resistance of the load, speed and the condition of your IP. 14-20 is the most it can be narrowed down in my opinion. A 5K pound led block in a M416 from Denver to OKC on I-70/I-35 might give you 20mpg if you ran 55-60 the entire trip. Turn around and gain that 5000 feet back would put you down around 14, maybe.