The reputable RV parks that do allow short term rentals, charge a management fee to manage the site since it is assumed you (the owner) are not going to be there very often.

Some even charge for renewing leases. 

I think your estimates on what to charge for rent in Hastings is low.

@Christian Beebe You're saying you spend $150k on average per year with credit cards for expenses on income properties and you're able to pay-off on average 95% of the card balances each month leaving 5% exposed to being charged interest each month

I would not take that owner financing if possible, I don't know about your market but that interest rate is double a good cap rate in CA and more than most hard money lenders would charge. 

I was looking at gas and water paid by landlord and thought I could charge $55 each for gas and water as flat every month fee.

In addition, the PM charges their "new tenant fee" equivalent to one month of rent ($1000). 

So if you put $100k in a savings account with them, they might be able to use that $100k as the basis to lend out $500k... and charge interest on it.

It provides more of a cusion for unexpected issues that come up.

(Would sleep up to 20 between the 4 houses)However, we ended up putting too much money into the properties with unexpected electrical, plumbing and roofing needs.