I think we're going to need a bigger boat! What kind of fuel milage did you get pulling that beast with the 8.1? 6?

I also pull my 37 AT with an 8.1 3:73 2500 Avalanche. And for some reason the mileage isn't as bad as I would have thought 8-10mpg towing. I have a friend that had an '04 2500HD 8.1 Allison 4:10 truck and it was like we were throwing money out the window 4-6mpg towing. There was never a gas station it didn't like. I do drive with a light foot and he has a tungsten foot. The best I have seen with my 8.1 was 16.2. (calculated not readout)

With my old 2001 3500 dually 4x4, 6.0 gas motor, towed everything from 35 ft tag enclosed, 44 ft gooseneck, 38 Top Gun etc. Always towed great with plenty of power. Bought the 47 fountain with triple 502's, the truck struggled pretty bad. Very long 700 mile trip back from Alabama with it the first time towing it. Finally bought a GMC 4x4 dually duramax/Allison truck a cpl months ago. Tows beautifully. The 6.0 motors really hold up well & last a long time, I'm a bit of a fluids maintenance nut though.

I used to get 6-8mpg towing my heavy flat deck with a 8.1l
4mpg stop and go .
It was always downshifting and 3k+ rpm trying to go up the smallest incline . I hated every minute of that truck.

My v10 averaged 10mpg regardless of pulling or not. It also had 4.30 gears in it.

I remember reading that years ago before I bought my mine. I just put it in 3rd and go. I'm probably lighter than you also.
Was yours 4:10's Allison?

as low as 7 as high as 14 depending on the terrain

I'd be interested to see how the new GM 6.6L in the 2500's tows/mileage. Heard good things but not from real heavy haulers. Love the new trucks and the amenities, just not the prices for the Dmax. I have a 2011 Dmax right now that I just became married to after a CP4 explosion, but I'm still on the right side of the NADA values barely. Maintenance on diesels is just expensive for occasional towing use. Feel more harm comes to it from sitting in the garage than actually driving it, my VGT vanes are sticking now.

I used to have a 8.1L avalanche, great truck and still miss it, but would have gotten 6-8 mpg pulling my 380 and would pull anything and just lug, just not fast like a dmax. Had 4.10 gears but the 4L85E trans, would have been awesome tow vehicle if it had Allison, even with low rear gears, 1st and 2nd trans gears were way too tall and far of a split for heavy hauling. Now if they still had the 8.1L or derivative with the new 6 or 10 speed transmissions, would be total game changer. Pulled my 380 for a bit with a 1500 chevy lease truck 5.3L/6-speed till I could get out of it, actually wasn't lacking for power and I'd say it got 14k load moving just as fast if not as fast as the 8.1L/4-speed did. Just not enough azz in the truck for brakes/lateral stability for that big of load. Got 9-10 mpg around town too pulling that.

I would not consider an in-bed auxiliary fuel tank for GASOLINE. I don't even like hauling additional fuel for my race car in the bed of my truck in race cans. Gasoline explodes, and you have no time to get to safety in a crash. Diesel does not explode as such.

Coincidentally, Titan XL fuel tanks sponsors me and my race car. They make the bigger tanks that go in place of your factory tank, and in bed tanks too.

I am putting one of their tanks in my Ram 3500 soon, and I have a discount and free shipping code for others to use, coming from them at some point.
https://titanfueltanks.com/
https://titanfueltanks.com/products

How about a 3.5L gas? Pulls better than most gas trucks I've driven. 32' Sea Ray or 24' enclosed with 69 Camaro usually. Set up correctly the Expy pulls both really nice.