I’ve seen magnolia beach rv park (which is in port Lavaca) mention dumps but you have to stay one night. They specifically state on their website they’re ok with you booking one night for $30 just to dump fill up your water and recharge your batteries (if they have openings).

We filled our tank with fresh water in Victoria, before heading to the beach.

Exactly what Jessica said. RV parks were not helpful as there was little upside to help dry campers. You could fill up a water jug from the sink in the restrooms if you felt comfortable with your filtering system. We carried extra water jugs to last the week. If you can do it, you should! Magnolia beach is a treat.

When we were there back in 2019, there was a small RV park down the road that allowed us to fill up for a small fee. They were super nice. I believe it was called Oceanside but I’m not sure if it is still there.

I have filled up at the marina.

We couldn’t find any place closer than Victoria to fill ours. None of the RV parks around seem to be willing to allow that or dumping. Some people mentioned asking locals who allowed them to fill but we didn’t try that.