carrying Atv in short bed truck with TT

Does the weight of the ATV plus the tongue weight of the trailer fit within the load specs of your truck? Perhaps you'd get the best fit of weights and space by putting the ATV in a toy hauler but that's an expensive solution.

your fight a weight problem you can,t win. just think of all the pounds hanging back there your front wheels might float.

Are the wheels supported on the tailgate? If they arent putting weight on the tailgate when it's down, you could simply remove the tailgate. If they are resting on the tailgate, a piece of 3/4 inch plywood, cut so that it only projects out the rear of the bed far enough to support the ATV. With the front wheels also being on the plywood would have plenty of counterweight to keep it from tipping. And you would just remove the tailgate altogether.
You would have to secure the plywood for when you are loading and unloading, and ensure you have the atv strapped down well.

I have 5ers so my bed is unavailable so I had a local fabricator build a very stout rack on the back. The supports are "sistered" to the back 3' of the frame.

That solved the space problem but I still have to mind how much weight I put back there.

I got to thinking about it when I read your question... I bought my first pickup in 1979. It was a long bed. I've owned pickups ever since. Every one a long bed.

1) remove tailgate and put two aluminum ramps under the wheels.

2) Use one of those big racks and load sideways over the bedrails.

3) build a ramp at the front of the bed to get the front wheels up high enough for the rears to fit on the bed.

Just three options I've seen used, I'm sure there's hundreds more.

My TT only has a tongue weight of 500 Lbs. and dry weight 5366. The atv ( weighs 700 lbs. ) tires sit on the crack between bed and tailgate with the back half of the tires touching the tail gate a maximum of 6 inches. My 2012 f-250 SRW crew cab power stroke specs say max gvwr 10,000 lbs, payload weight rating 3290 and 3650 . I'm not sure exactly what that means but seems like the max weight on front and back of truck ??

You might be able to use a sideways carrier. Though at $2,500 I would try the 3/4 inch plywood idea first.
https://diamondbackcovers.com/collections/atv-sxs-carriers#products

The truck bed is actually 6 3/4 ft long but I have an auxiliary fuel tank that takes up 18 inches in front. I had considered putting a couple of 2 x 12 boards under both front and back atv tires that should hold the tires. Anybody done that ?

First, hook up trailer, lower the tailgate, drive around the block making right turns. What is the question?

Well, now I feel like an idiot !! I didn't actually drive with the tailgate down. I just looked at the trailer with the tail gate down and it didn't look like it would even be close to clearing if I turned. Off to the TT I go !!!