Yeah, I think Larry is right. A number of times, particularly with older parks and more so older Canadian or Montana ones, we’ve had issues with either the seemingly new but bargain-basement outlet at the post, an overloaded master breaker for an entire section of a park, or a simply no longer adequate entire park system - even with 50amp posts.
So yup, more likely with 30’s. You may see nearly 120v for awhile, then enough motors throughout your park section suddenly hit congruently and cyclically enough to drop your supply, and your coach self-survival kicks in. If I were betting, based on my personal experience I’d put money on the issue being your Park, not the coach system.
Joel