Based on your details she is saying is that 18 lots are occupied over the course of a year for $300 per month (using your 65K gross number), but she covers utilities. Depending on the expense ratio you estimate (I would probably say between 50 to 60% based on current operations) that the valuation is closer to $250K or less. The past water and power bills may offer some indicative amount of occupancy over time, but I would probably spend several weekends out there over a 3 month time period to see how actual occupancy stacks up with the collections, assuming it's a good deal worth your time.
Separately, Frank and Dave offer a course on RV Park investing too. I haven't seen it, but given the quality of their mobile home offerings I suspect it's worth a try: