Well, I couldn't catch this guy, but while I was in Beatty NV, I saw a 5th wheel RV go by with two axles, and duallies all around.
I had to look twice; sure enough, four tires per axles, on the trailer.
I have never seen this before so I thought it must be a custom.

 

Newmar has been using them for a while

 

I've seen them out at Qurtzite,AZ. i assumed they were toy haulers needing the extra payload.

 

Newmar has stopped making fifthwheels. Nice rv's.
Labarn

 

I saw one at the RV show a week or two ago. I didn't really go look at it as it was obviously not in my price range.
must be some awesome load carrying capacity. Probably a little easier to turn than triple axle trailers.

 

http://www.newmarcorp.com/models/fif...Kountry%20Aire

 

Newmar had that setup on their big fivers for quite a while. It was a toss up between that and three axles or go to H rated tires on two axles. Biggest drawback on the dualies is the frame is moved in closer together making the suspension points closer together. Net result was the fiver leaned more in turns. Some owners complained of feeling less stable with it - - saw it swaying more in cross winds and dips in the road. I sure didn't like three axles, so my vote would be H tires on two axles. Newmar made some nice units.

Bob

 

Just fyI I have been a camper for years and run BFGs 10ply comercial tires they are made for the heavy cube vans I call them a steer or rib tread on 16"rims under a cedar creek I found them in stock at sams club they been trouble free 2 years going south from Canada and home again Sams will not work on a 5er so I bought tires and had a truck shop mount and spin ballance them...John