I assume you have vast understanding of the market where you want to build and what homes sell for, what the inventory supply is and demographics.

What's going to happen to your rents when there is a ton more supply and your unit is the outdated one?

It will be a detriment to many tenants out there and of course make housing supply less attractive 

Where I struggle is sacrificing a locked in low rate for higher variable rate.

You supply a place to live.

Housing prices in the low $90k doesn't bode well for a park.Moving to the supply side, how many other Mobile Home Parks are in the area?

Lot of variables in this question but I'll try to keep it simple.

I'd recommend using the Bigger Pockets Calculator:  https://www.biggerpockets.com/buy-and-hold-calcula...It does a decent job of breaking down the cost and income variables and calculating the key performance measures for real estate investment.  

any up front fee other than credit or appraisal is not warranted by any lender unless your doing a large commercial deal where there is a huge underwriting job to be done.. again not with vanilla SFR or small multi.Of course owner occ variable rate loans can start out low.. but I suspect that's not what you were doing.these guys prey on those that just don't know what they don't know in the mortgage world and just send in the 870.00 and never hear from them again.. sad but very true.also if you like American Greed there Is a great episode on Remmington Financial I highly recommend these guys were the king of mortgage fraud and pump and dump due diligence schemes.. they got away with it for 20 years.. 5 to 10k at a crack..

a conventional mortgage is the cheapest loan you can get. a heloc will be cheap, but have a higher, usually variable rate, and much harder to get on investment propertya personal loan is most likely not going to be a viable option. unsecured personal loans have high rates and require high liquid capital