I built one for when I had my pop- up... I called it "The Centipede."
I required pneumatic tires instead of steel casters so as to be able to roll it on a gravel driveway outside of the garage. I had to fit it through a door height of only 7'-0" so minimizing the dolly height was VERY critical.
I made it out of 2x and plywood and had about 2" to spare at the highest point on the camper.
One item to note: Place camper COG in the center of the dolly. This may not mean that the floor base of the camper is centered on the dolly. The camper may need to slide back of the dolly and have some of the rear of the camper hang off dolly platform.
It did OK on my garage floor... small pebbles were no issue, but when rolling it outside on gravel, I ripped a 4x8, 1/2" plywood sheet to lay down to make rolling easier.




Editted for speeling and too ad adishunal fotos.