Welcome to iRV2. The rattle from the back when shifting could be anyone of a number of things.
Some of the things I would look at are:
Exhaust too close to a frame member'
Exhaust support broken
Intermediate drive line support loose.
Rear transmission mount (bracket loose)
If it is a clunk, rear axle or ujoints worn.
More than likely a broken motor mount would be heard at the front.
Ain't owning an RV a lot of fun...
Now when you get it fixed....
Ken