Are you talking about something like this?
http://www.trailertoad.com/
This looks like a complete system to improve your weight distribution. So the hitch weight of the trailer goes on those 15" wheels, not the hitch on your RV. Then only a fraction of the overall hitch weight is put on your RV's hitch.
The system is pretty sophisticated. It has locking pins to allow backing up, that are raised for driving forward and allows some steering of the tires, less tire scrub. When locked, it keeps the tires straight, so you can park the trailer and back up, hopefully at speeds less than 10 MPH.
I know with my 30' motorhome and 190" chassis, there is only about 12" of frame extension behind my factory frame. Your length indicates about 24" more frame extension, and you stated that somebody already looked and made it stronger. That is a good thing.
Your GCVWR should be 26,000 pounds. Mostly limited by the strength of the transmission. I would recommend installing a larger trans cooler in your case.
When I towed my brothers cargo trailer, I did not use any tow dolly like you are planning. I as fine because my RV has a 100 gallon rear mounted fresh water tank under the bed. Leave it empty, and you have about 800 pounds of extra hitch weight available. This hitch weight takes away from my front axle being marginally overloaded, so it works out well. I have extra capacity on my rear axle, but not much on the front one.
The extra two tires will not provide any extra braking.
You might want to look up disk braking systems.
ETrailer.com Good luck in your travels,
Fred.