4WD has its value, but whether you would ever need it or not is guesswork. When we towed 5W's, I got stuck three times in one summer towing with a 2WD + Limited Slip differential. Wet grass was the culprit once, muddy grass another, and loose gravel the 3rd. Two of the 3 times a neighbor with a 4WD truck helped me out. I darn near traded trucks by the end of summer! But when I got over the frustration, I realized I had never been stick before. And now I can say I haven't been stuck since either. Sometimes crap happens.
But if you drag your RV into wet areas, sandy places (like a beach!), on deeply rutted roads, or into the back country where help would be hard to get, I think having 4WD would be well worthwhile.