I have a 3/4ton dually step van Alu body low miles & in very nice shape. Has anyone hear ever towed with a setup like this? If so how was it? How big of a trailer did you have?

I had a 1970 Chev step van.....it was more like a 2 ton though.
Mileage was non existant.......it had a 350 and some weird transmission that had no park in it , but plenty steep gears.
I hauled dirt bikes around in it for years and it worked well........just thirsty.

My motorhome is on a P30.....

That's the one as I recall.
But the motor home transmissions have park........mine was a three speed auto with no park and a drum brake on the back of the trans for a parking brake.
It never gave me any problems so I never had to buy parts for it......the trans , that is.
The motor liked to crack exhaust manifolds every four or five years and center dumps were getting hard to find.
Other than that , it ran like a Thoroughbred for seven or eight years.
I bought it from a small town fire department with 20,000 miles on it in 1985........it was their rescue squad and most of the miles were just to make sure it ran.

My truck was owned by the police & had SWAT on it. I need to finish taking all the gun racks out of it.