you're probably going to encounter "shared" hookups...one water spigot for every X campsites. X is usually two but can be more. one municipal park (now closed) that we used to frequent had 6-8 water bibs on a pipe. folks would lay up to 50' of water hose to reach "their" bib from their campsite. if they're telling you to bring a "Y" that means there is either one bib for two campsites or a large number of campsites fed by a smaller number of bibs. the 'Y' will allow two hoses to be connected to a single bib. any garden supply shop like home depot, lowes, ace hardware, etc. will have these. I prefer the metal ones.
more unusual would be sharing an electrical post with more than one other campsite but we always carry an extra 50' of 30-amp cable with us just in case.
any RV supply store will have a 20-amp adapter for your 30-amp cable. if you have a 50-amp coach you'll need a 50-amp female/30-amp male adapter and then a 30-amp female to a 20-amp male adapter. like karl malden used to say...don't leave home without it.