This is a very interesting topic for me. As we were looking at either purchaising a DP or going with a new truck and 5er we too were wondering about length restrictions after hearing the horror stories that you will never get into this or that place if your over 34'.
There is an RV Park on the beach we like to go to but they had a 37' max length, well while we were searching for a new/used RV they actually changed their max length to 40'.
We did reasearch this pretty extensively and really didn't find length restrictions with a few exceptions. What we did find was a lot of knowledeable RVers, that shared their experiences such as height being an issue as mentioned (trees) and when length played a factor it was more in driving to some parks on narrow winding roads. But time and practice should overcome that possible issue.
Buenos Dias from New Mexico
Stephen & Nancy
2015 FR Widlcat 344QB 2014 Ford F350