I'm going to make a guess that you are active duty. I'm retired but I agree with much of your concerns. I think that any one on TDY should be allowed to stay for the length of the TDY. As for PCS, I think from 3 to 6 months is reasonable. That would give them time to search the local housing market. if the area is an area that is hard to find housing, allow a 3 month waver (up to 2 for a total of 1 year) but require they move to a new site. Many of the large civilian campground have stay limits but will let you move to another site in the same campground. Knowing they will have to change sites cuts down on the add on or piling up junk.
For the retire and others limit a stay to 1 month and then require them to change sites. Only take reservations 2 weeks in advance unless they have TDY or PCS orders.
It would be good if all branches were run the same way (uniformed services after all) and even better if one place controls all the campgrounds. For example: this website could setup a small phone center to take phone reservations, and take reservations on the internet. That way the local manager or a camp host could be the onsite enforcer. This would insure that, except for TDY and PCS campers, every site would be available at least once every month and effectively stop homesteading.
