My 5th wheel is 19 years old and maxed out weight wise (15,000lbs). We travel an average of 4800 miles per year. I have never needed to have an alignment. All 4 tires wear evenly.
There was an issue after we had our axles flipped when the trailer was new. The factory service department failed to add two cross braces. On our first trip after the swap, one of the axles shifted out of alignment 1". I noticed one of the tires stuck out further than the other. We took it back and they added the cross bracing, no problems since.