Hi Bo and welcome aboard. Hope we can help a bit.
Haven't been to PINS so can't help there. As far as Mustang Island goes, we've been there once but didn't stay. Saw nothing wrong with it. Just not to our liking. RV camping is just two rows in a straight line. Kind of like camping in a parking lot. Beach camping would probably be OK. I assume there is some kind of limit on the number of beach campers and if you don't already have reservations, there will probably be no sites/room available. If you decide on Mustang, I would definately give them a call and make sure you can get in. Here's a site address that should give you the info you need:
Mustang Island State Park
Many like Mustang Island and as said, we saw nothing wrong with the park. We just have different ideas about camping. No matter where you end up, I hope you have a great weekend. I envy you some. DW and I have to work and we know there will be no last minute reprieve

. Again welcome aboard and as always,
Happy Camping!
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Turn Key, DW and Pepsi & Cherry, The Camping Boston Terriers
'03 Chevy 2500HD, 4X4, X-Cab, Long Bed
'04 K-Z "Durango", 275RK ("Sunday Haus II")
Twin Kayaks, "The Ride" by Wilderness Systems
North Central Texas, Where The West Begins!