There has been many pro's and con's of having a tag axle. When I was at Newmar a week or so ago I had the opportunity to crawl all over several bare chassis's, the K-2, the MM GT tag, and the MM GT single rear axle.
With some recent posts regarding u joint failures and issues it was apparent that both lack of lubrication and angle of operation were the cause of premature failure.
I really don't think the lubrication was the cause, but more the angle of operation. With the suspension going up and down while driving and moving a coach with the air down the angle of the u joints cause considerable stress at the joint. This stress in not apparent with a tag axle due to the substantially longer drive shaft, four feet longer than a single.
Even running down a bouncy road the longer drive shaft reduces the movement angle to about 25% of what the shorter drive shaft requires. The longer shaft can also handle moving at full down position without any stress what so ever. Many singles are restricted to very little movement and even at that may sustain damage if any torque is required.
The last coach I had was a single, DSDP. I never had any u joint problem in the 58k miles, but I looked after it. The longer drive shaft however will probably never suffer from u joint failure regardless of reasonable driving treatment.
Just thought this was of interest.
Peter - Doctor of Mixology