hi fred...
fairgrounds (especially near larger cities) often have 100s of sites and buildings for group events.
some mega rv parks (that are not part of fairgrounds) also are used for larger rallies.
often the big wb events, construct ALL of the essentials (water/ele/dump) even in big open spaces, then use the buildings near by...
there are group camp sites in some state parks (150 units might be pushing it)
and we had ~80 units and 100s of people at THIS state park....
http://www.airforums.com/forums/f288...ast-32154.html
and ~50 units/150 people HERE, but could accommodate 100 units with proper timing...
http://www.airforums.com/forums/f288...lly-35478.html
church camps, boy scout camps, the hugh hefner nude dude ranch, and so on host big trayla gatherings...
you can LOOK at the 'good sam' website and see a list of their PAST events and locations often 1000s of rigs.
________________
you asked a vague question WITHOUT any background or focus...
get more specific and 'splain what is UP and detailed answers may accumulate.
cheers
2air'