I do ALOT of towing 5er and bumper pull and have yet to need 4 x 4. I will say that I do not go into campgrounds so that maybe an occasion it would be helpful. If road conditions dictate 4 x 4 while towing then IMHO it's time to get off the road and call it a day.
You are correct 2 x 4 2500/3500 trucks are hard to come by but they are out there.
1990 Jayco 250 Special
2008 Ram 5500 DRW 6.7, Cummins, Aisin auto
Holland/Binkley 5th wheel, Reese Dual Cam, and more
232,500 kms in 2013
200,000 kms in 2014
160,000 kms in 2015