My two-cents-worth. That $500 up-front fee will pay for quite a few summer RV park/resort sites without the limitations of a membership OR the 70-100 buck annual fees. Only been full-timing for 18 months, but have NEVER been shut out of an area we wanted to stay, because the only game in town was a Membership Park.
As you travel, you'll likely stay one-niters in some places, and weeks in others as a base-camp. With a membership you tend to limit yourself...because you've already paid, so-to-speak.
For example: No membership parks in Astoria, Oregon. An amazing place, packed with history (Lewis&Clark, Columbia River, WWII...). We would NOT have had the chance to stay and soak up the cool weather and all the museums...if we had to stay in a membership park in Portland or Seattle.
Now, we stopped 'camping' a long time ago. No longer take our larger/heavier RV off-road and tend to seek 50amp full hook-ups with a clear view of the southern sky (satelite TV). Just my humble opinion that numerous 'membership' parks have fallen below standard on amenities...
Why not check out several of the membership parks you're considering in your travels next summer. They SHOULD all accept you with open arms as a potential
member. THEN make your decision.