I think there are 3 different situations:
1. Private parks/campgrounds that allow you to pay for a period of time, park your RV, and come and go as you please.
2. Public campgrounds that do not have rules about occupation.
3. Public campground that have rules about occupation.
Is it selfish to "hold" a campsite and not occupy it? Well it depends on which of the above:
1. No. The private company promotes and encourages this behavior. It is desirable in their mind.
2. Yes. It IS rude, however it is NOT against the rules. The rudeness lies in the fact that you are taking a public resource and locking it up so others cannot enjoy it. It IS selfish because your desire to get a "decent spot" for a weekend takes precedence over others' desire to use a "decent spot" for the time you have it locked up unused.
3. Yes. AND it is against the rules. Not only are you being selfish and rude for locking up the campsite, you are being selfish and rude because you have decided the rules don't apply to you because your desire for a "decent spot" for the weekend trumps everyone else's desires.