Assuming your test ad reveals good demand, and that the low occupancy is solely because of mismanagement...
Your expense ratio should be closer to 50% due to low occupancy, private septic, and already smaller Park. Will the new septic handle up to 24 homes or will that be an additional capital expense? Also 75K on an acre in rural east Texas is rather expensive. That's retail prices and you may only be able to make money owner financing that holding the paper. Is that on it's own septic and well also?
The Seller is clearly pricing in the value of the POH rent so he won't understand any of your logic with a low offer. You should try anyway and just follow up every 45 days with the Seller or Agent until they see the light or find a sucker. I would be closer to 225K for this deal, and you would be lucky to find anyone to finance it. I would move up some with attractive owner financing terms.
MHU has a service to get their formal opinion on a deal, and it's worth it: http://www.mobilehomeuniversity.com/mobile-home-park-investing-books-and-courses/mobile-home-park-quick-deal-review.php
Frank chimes in from time to time on the forum, but if you're looking for more than that the above is the route to go.