I don't know that there is a definitive answer. Now we have a 30 ft. Class C MH and did have a 26 ft. 5th wheel. I would prefer to stay around 27-29 ft. It's not that you can't get into any state and national park sites as it varies a lot. In many of those campgrounds there will be some long sites and some not so there might be 6 sites left and none suitable for over 25 ft. but many longer ones already filled. Shorter is better in many situations such as gas stations, parking lots, etc. Traveling with a friend with a 35 footer firmed up my opinion when I compared where he could go and where we could with our 26 footer.But many will swear that there is no problem at all with any length.