Good Morning Chris Franklin,
On my V-10 cooler the horizontal pipes are 2-3/16 OD with the exception of the two 2-3/8 horizontal pipes at the rear of the cooler.
Because I am pulling gooseneck trailers and to keep the trailer clear of the cooler I ran a single pipe ( vs. two or three ) on each side of the truck
The vertical pipe I used is 2-7/8 OD.
If you can find 3 inch square tubing for the vertical part of the cooler it will be much easier to build. (Take a look at Chris S. cooler with the vertical 3 inch square tubes.)
(2 inch PVC pipe will fit the 2-3/8 OD steel pipe)
I have some 1-7/8 tubing I will be using for the horizontal part of the Dakota cooler I am building.
You may need to hook to one of your trailers and judge where to stop your cooler rack so you will be able to make sharp turns.
I have the trailer hitch located over the rear axle and have 64 inches of clearance from the rear of the truck to the cooler.