I'm on my second season of towing with my V6 '09 Pathfinder and it has been great. Yeah some steep hills can be a challenge but I slap the shifter over to 3rd gear and it's not a problem until I level back out and put it back to D. Regular Highway driving and backroads have never been an issue in D and it's never had to hunt for gears.
My Jayco X19H is around 3600# dry and with our gear we are sitting at roughly 3900-4100# depending on the length of the trip. I use a WDH Hitch with Sway Control and it's been issue free.