It sounds like a good deal, especially for a local plumber! Get it under contract for $200k if you can and then you can dig deeper.
Filling lots of very time consuming, that will be the management headache for this park. It shouldn’t take two guys to manage twenty lots and 13 acres but if all you have to pay is $300/mo and they are happy to work together it might not be a bad situation. If there aren’t any major issues with the park and the clay pipes are in decent shape, this is a win.