I have no idea about the fuel consumption. But if you will have a Truck mount and fresh water tank I recommend the 5.3 liter or the 6.0 liter. I have 5.3 liter truckmount van and it has great pick up no matter the weight i'm pulling.
I would never choose the 4.8 in a 3500, the weight alone with a truck mount and it would struggle. It would probably burn more gas cause of that. The 6.0 has been put in GM's pick ups for years and been a proven work horse. This is what I have in our 3500 extended. The Difference in gas burning will be minimal compared to maintenance cost of repairing a beat 4.8
I just wasn't sure how they were in the vans. I have an 03 crew cab 2500hd 4x4 with the 6.0l and it doesn't do that good on mileage. It will however pull pretty much anything you want. I once had to pull a two ton grain truck loaded with corn about 4 miles to the in-laws farm. It was a load on the truck but it managed with no issues.