Hal,
If you want a detailed, drawn to scale layout of the park without having to purchase expensive engineering based software. I would suggest you do the following.
Rent Manager may have an option to draw a layout. I am not aware of the cost or capabilities. I also do not know if it will give you any kind of proper scaling.
Most photo editing/design software would be capable of doing what you want done. It would just take some time and practice to get it done. For example, I use photoshop and I can start with an 8.5 x 11 template. Then do the math to get my feet - inches scale. From there I can draw a layout of a property for example that would have scale and be pretty accurate. It would however be time consuming. I think doing this to a 50 space park with utility lines/connections, etc. would take me somewhere between 8-15 hours.
There are other programs out there that are open source (free) or inexpensive that are designed for drawing schematics for electronics. These may work as well but I have no experience with them.