As far as I know , an outside shower does not mean its not self contained. You have to take a dump so they got you there but you don't need a shower when fishing
so I don't think its a problem. They may say you can'y use it but I don't see why there would be a requirement.
You could find out what's Ok in RI by calling Burlingame Park. They handle the sites in Ninigret and those require self cantained vehicles. I can't help you for MA.