On typical 3500 box trucks, your available payload is close to a 2500, typically 2000-2200lbs. The weight on the longer frame + the box typically brings the weight up quickly, wouldn't recommend a 3500 box truck. I have an isuzu npr 3500, 14ft box, light cabover truck, 2000 lb payload available with full fuel tanks + me in it.
3/4 tons have the same payload once you stick on everything the manufacture pulls off when they do the weigh in (bumpers, radio, ect) and you add fluids and people in the weight.
250's are great for pulling trailers, wouldn't bother with putting a tank in the back. Doug's setup up there is cold (burner add's 500-600 lbs of weight most times) and he can't run with the tank full. Fine if you can fill your tanks at the work site, but you are limited on your jobs.
450/550 is the truck I would put the weight on, ie tank + pressure washer.
250 run around truck + trailer puller.
Here is my experience, dodge 3500 dually, 270 imp gallon tank + base line pressure washer + 240,000 btu burner (I lifted it in the truck, its that small) 11500 lbs full of all water, diesel, gas + me.
Ford E550 2002, 20 ft box 10,500 lbs with the 54 gallon fuel tank full, me sitting in it, no equipment in the back. I have a heavy pressure wash setup, easily double if not triple with all the fuel weight then most. I can only fill 2 x 275 imp gallon tanks and them I am at my 17,500 lb limit.
That is still 5000 lbs of water weight, keep in mind most tanks are not measured in US gallons, but UK gallons. Difference is that a US gallon is .8 pounds, UK gallon is 1 pound.
I am just talking from what I have, trucks and payloads can be all over the place depending on different factors. I had a mini heart attack when I first weighed my NPR with the water tank I had in it. I didn't understand water weight when I first started and the 600 gallon US tank and pressure washer setup put me way over weight. I upgraded to my big truck (Tandem freightliner) from there as soon as possible and have made sure to keep my trucks legal weight wise ever since.