Unfortunately, there is no option to assign attributes to accommodations, only to accommodation types. As an alternative you may group your accommodations under accommodation types with 30AMP and 50AMP hookups respectively, e.g.:
RV site (30AMP hookups): RV 1, RV 2, RV 3 etc.
RV site (50AMP hookups): RV 1, RV 2, RV 3 etc.
Another option would be creating a separate accommodation type for each accommodation, so you could set desired parameters to them.