I would not go with any truck smaller than a 1 ton (Dodge, Chevy or GMC 3500 or Ford 350), With that much weight you want to put in there, I would probably go with the larger truck like a 450/4500.
Gas engines are good but most Diesel engines will get you more mpg, especially on the highway.
I have had great luck with the Dodge Cummins 5.9 Diesel in my 2500 truck. I can put a pallet of drums weighing about 2100 pounds in the bed and it does not squat more than a couple of inches but I don't keep that much weight in the bed all the time.
You can find the truck in a single real wheel instead of a dually, just have to look a lot more as they are not as common as the dually trucks are, you can put some more weight in the bed of the dually trucks, I think about 500 to 700 pounds more if I remember correctly what they told me at the dealer.
I am looking for another truck to do something similar but only need about 2000 to 2500 pounds all the time for truck washing, holding drums of chemicals, small skid, small water tank, hoses, reels, etc..... not many people have experience pulling or backing up trailers and there are some places that it is hard to get into with a trailer.
You can probably do the same thing with a decent box truck, great room for advertising on all sides, keeps things out of the weather, theft deterrent, but there are blind spots on the sides (get the round mirrors to help) and get the backup camera to help when backing up or to look behind you). You can get the box trucks all over the place like U-Haul, Ryder, Penske, Budget, etc.....
Most of the trucks are rated for only so much weight in the bed of the truck but most of us put more at one time or more or sometimes all the time. If you do plan on that much weight all the time, make sure you have a shop go and install helper springs or air bags to help with the added weight along with going through the brake system and making sure everything is in new condition as the truck will be harder to stop with all that added weight, if you buy a truck from up north and there is some rust, have the brake lines replaced as they will give out on you when you are driving, been there and done that, they don't cost that much but will save a life or more.
Securing everything in the bed of a truck can be done but it is easier with a skid unit like the water dragon type where everything is secured on a skid that can be installed or removed with a forklift. One of the best things about this type of skid is that if your truck is down, you can rent a truck and slide this into the rental and keep working until that truck is out of the shop if this is your only work truck. Bolting everything down into a truck will take a while to get everything out and you cannot secure it properly into a rental.
Just some of my opinions, hope this helps some.