For seasonal camping, you really need to offer the basics: electricity, water and sewer. All the other stuff is fluff. Without those three items, it will not be enticing for folks to stay 3 months.
You could skip the sewer since there is a dump station nearby, but if they have to drive their RV to get fresh water every 2-5 days, they aren't going to bother staying for a summer.
If you can get those three things worked out, then:
Fire pits and picnic tables are considered standard in most rustic pay campgrounds.
Don't bother with gas ranges. At the most install those charcoal grills on posts that you see in parks.
No storage of gear beyond the season - you don't want the liability.
