No more than $20
The price should be less than running your genny for the night. Must offer services you can't get at Walmart or others such as washer/dryer, hot and clean baths, water, sewer (don't have to be at every site), maybe video rental, etc. I can see charging by the hour, so might pull in at 11 pm and leave at 6 am, others pull in before 4 pm and leave around 10 am. Must have pull thru and easy in, easy out, close to the freeway and gas station. Fast and easy way to pay, maybe without even having to get out of your rv and even after hours. Maybe next to a fast food place that would make deliveries. Also a dog walk. If in the south, SHADE.
NO DISCOUNTS, same price for everyone, keep it simple.