You NEED an engineer for this. Sizing water mains in such a situation is not something for an amateur. I can tell you that you will start off with really big pipe and work your way down to much smaller sizes. Suggest you allow for each section of the park to have its own isolation valve in addition the valves you will need for each site and one for the entire park. This allows you to repair parts of a system without having to cut off everyone’s water.
Make sure to ask your engineer if there is any benefit to “looping” the pipe. Without going into a detailed discussion of hydraulics, there are certain benefits to this approach and you may (emphasize “may”) be able to use smaller diameter pipe. Pipe gets real expensive quickly as the diameter increases. The cost of pipe changes constantly with the cost of oil so there is no way any contractor can even think about ballparking this without detailed drawings.
You will most likely be using CPVC pipe. Make sure to add a metallic strip on top of the pipe so that it can be located with a metal detector. Insist on the contractor supplying “as-built” drawings upon completion or mark any deviations yourself on the plans prior to backfilling the trenches. Things have a way of varying dramatically from those nice drawings the engineer delivers.
Do you have extensive landscaping or will you need sprinklers in various areas? If so, plan for “stubs” with their own isolation valves so that you can connect one or more backflow preventers for an irrigation system. If you plan ahead, adding this later is easy plus you will save a ton of money. You may even want to consider laying in control wires to these stub locations and running them back to a central control point.
What about future electrical (either high or low voltage) needs? Trenching is expensive and it’s easy to lay in conduit now rather than later. If you install extra pipes in the same trench, make absolutely certain before you allow backfilling that there is a real space between all pipes. If you ever have to dig up a line for repair, it’s impossible to cut into it if other pipes are in direct contact.
If you trench across a road, put the pipe inside of a “sleeve” so you can add more pipes later if needed. E-mail me if you like and I will explain why.
How do I know all this? I’m an irrigation contractor (amongst other things) and this is what I do for a living. Learned all this wonderful stuff the hard way. Irrigation systems are actually more complicated than potable water systems.