I hope you're able to find a solution.

If her concerns are reasonable, then she's a valuable asset to the team, thinking critically and pointing out potential issues you may not have thought about so you'll have solutions ready if they arise. 

I would just monitor the situation  From my experience as a PM tenants and neighbors always want the landlord to settle their differences.

It is not a deal killer, but certainly presents risk when the entire park could be out of a sewage solution if the septic has a problem.

We are also looking into going the Airbnb route for vacation destination spots around the country, but I can make that a separate post if need be.Does anyone in the community have good experience/suggestions for areas with companies looking for housing solutions?

That was my solution to a similar question and happy to talk you through the details if you are interested.  

You need to document the complaint, the issue, the solution and all the times she has refused access.

He is one of the sharpest lenders I know, and sounds like something he will have a solution for if you want to pick his brain.

What is a cost effective solution here?  

Once you do decide to procure your first rental, definitely lean on the BiggerPockets community as a resource; I'll also say that it could be worth considering a shot at a software solution built for DIY landlords (1 - 15 units) to bring some automation to the rental management process (just to make things much much easier for you)!