Pools and hot tubs generally are most desired by families and people on longer stays. They are double edged swords. They are costly to build and operate, they are labor intensive, they are liability hazards, the hours and dates of operation will NEVER please everyone and they tend to draw out bad behavior.
The good side is they are an amenity that adds value to some travelers. I have found that only families with kids seek out parks with pools if they are only overnighting. Is this a market you wish to serve? Before you answer, realize there is no wrong answer, and no matter what you offer it will turn off other potential guests. For every rig with 5 caged up kids needing that pool, there is a rig with two retirees who choose a park they feel will be peaceful and quiet. For every guest who chooses parks with many amenities, even though they do not use them, there is a guest that will avoid the parks with those amenities because they feel they are paying for things they will not be using. Trying to please everyone will lead to pleasing no one. I believe you have to decide what market you wish to serve and cater your park to those customers, be it overnighters, long term stays, families, retirees, vacationers, workers etc. etc. etc.
Several people have suggested you start very small and grow organically. I tend to lean the other way. If your goal is to have a full service park full of amenities, and charge a rate commensurate with those amenities, you need to have most of them in place when you start or you won't be getting that organic marketing so necessary for success. A review stating it is a great, cheap, bare bones park won't help your reincarnation of Disneyland dream come true. A missing amenity will always be considered by some to be missing even if you add it down the road. Similar to the old construction saw of "measure twice, cut once" a RV park should be built with "Plan, re-plan and then repeat the process" before a single shovel of dirt is turned.
