5th wheel length - trailer length - RV length?

size is what you determine is the right size. if you want to go to lots of older parks a used Montana 3150 would probably be a good bet....about 33 feet long.

We have a 39 foot 5th wheel and generally don't have any problems but we don't go into the old federal/state campgrounds.

you've been out camping so you should have a decent idea by now on how the sites and what you want work. you can always park your truck elsewhere if site too short.