raising 5th wheel

Using tubing I would worry about it crushing and the bolts coming loose that would be a rejectable condition and if the tubing is distorted that also would be a rejectable condition.
You could just cut out the stops at the back of the deck plate and replace the slider with a taller one, they make them in a lot of different heights.

 

That' what I did on my last truck, raised the whole slider assembly. I used 3" X 8 " 1/4" wall tubing & boxed the ends. I think the bolts that hold the slider on are 5/8" so when I drilled the holes in the tubing I drilled them just big enough to put 5/8" ID bushing stock in them. Never had any issues & did it for under $250.

 

...this thread looks oddly familiar...

 

http://www.fmcsa.dot.gov/regulations/title49/section/393.70

 

did one like your saying it worked good never had any trouble working lose or with DOT, but the next one may turn double back flips when he see's it, you know how that goes. DROY that's on here sometime told me ergon wouldn't let them pull a tank like that.Seen beenjammin's post might not be good idea anyway

 

Don't see anything there that would prohibit a person from fabricating their own riser, so long as it meets the standards called for in the regs.

 

It would be way less effort to just get the proper legs for the fifth wheel. They make then in all diffrent hights. Go to local truck salvage and buy the correct one.
Look much more professional and proper.
Just blow the stops off with a torch, slide old one out and slide new one in. Now would be a good time to put a kit in your fifth wheel while it's off. Done!