My answer is to use the $/sq ft method, and then add some padding.  

Hello BP, I am considering purchasing a property with two, two bedroom apartments and two mobile home pads.

There are a few Netflix shows that are full of information and are entertaining about owning and running a short term rental property, Like "Cash Pad" and  "Instant Hotel" that takes place in Australia. 

Besides, when you get "bigger," it's nice to walk into a bank/credit union with two previous years of professionally prepared tax returns to slap on Scrooge McDuck's mahogany table to get your loans approved instead of some cheap photocopied big box store tax software and legal pad scribbles.Blessings.

It's currently listed at 11% cap rate for storage units, short term rentals and the RV pads.

Pad their estimates by a lot so they don't have to worry about overagesBest case, you're going to pay the same as if you did a detailed scope of work from the start (since they'll require one) and worst case, you'll overpay. 

I have heard of owners setting J-Bolts in the cement pad when an outdoor compressor is placed and then drilling holes in the bottom of the unit where the bolts come up from the pad so that the unit can be bolted to the pad from the inside.

Take your time, get enough sanding pads, and you can knock it out if you want to. 

They will pad out the quote based on what the contract can not be certain of until they open a wall or otherwise start the demolition work.

I believe there is room for appreciation but when it comes to renting you will most likely be braking even or having a very small profit.