RVIA is not a "regulatory agency" -- it's a trade association.
The best response to RV parks that want to enforce an RVIA-only policy is to avoid them. That being said, sometimes it is nearly impossible to do so.
FWIW, we have stayed at such parks (as well as parks with a 10-, 15-, or 20- year old requirement, none of which we meet) and have never been asked. This is really a matter of them having a way to refuse entry to any rig that does not meet some subjective standard of "appearance."
It is worthwhile to note that NONE of the major bus converters is a member of RVIA, and so you will not find an RVIA sticker on a Marathon, Vantare, American Carriage, Liberty, or nearly any other Prevost conversion (the one exception might be Country Coach, which was an RVIA member). Yet somehow I suspect that none of those conversions is ever denied entry to one of these parks. I doubt, however, that pointing this out to the clerk at the registration desk will get you anywhere, even if you can wander through the park and find ten non-RVIA rigs in there.
It is discrimination, pure and simple, but there is absolutely nothing you can do about it (unless you happen to be a member of a "protected class"), since almost every private business "reserves the right to refuse service" to anyone for any reason.
Unless your coach looks like the Joads' truck, or maybe an unpainted schoolie, I would not worry about it. Besides which, you can't really do anything about it, since membership and the RVIA sticker are available only to bona fide RV manufacturers. You can, as Bob suggests, buy a wreck just for the RVIA sticker (although they are very hard to transfer -- they are designed to self-destruct if removed), but this is, of course, illegal.
If asked, I would (and have) simply answered that my coach meets or exceeds RVIA standards.