Definitely look behind the sheetrock to see what is there.You can use a 'see-snake' without much damage to the wall--about a 2" diameter hole.If you see the back of the other apartment's sheetrock, you've got a problem.  

There's multiple styles of flappers, the flat ring in the Mansfield, the rubber flappers with the egg shaped cone on many toilet brands, then there are different diameter ones too.

Another issue is that the liner diameter is not big enough to have 6 appliances on it totaling 300K BTU.

If these are in mint condition... the electrical will no longer meet code, and I recommend re-wiring.The plumbing is a toss up, sometimes we scope lines and they are clean as a whistle; sometimes we scope lines and a 4" pipe has an effective inner diameter of 1/4"... just depends. 

Use a small diameter foam roller and foam brushes with oil based primer per the earlier post by a pro (I agree, zinser and Benjamin Moore make good ones)and throw the roller out afterwards.

About 1/16” - 1/8” diameter dots.

I’m an out of state investor and looking at the photos I have, it looks like there’s maybe two stumps maybe 8 inches in diameter or so.  

The dimensions are typical, at around 19" diameter. 

I’ll find out how many homes can be on a Well and what diameter of a well I will need and if each trailer needs it’s own septic or can an X amount of trailers  share a Well and septic.

It’s only about 2’-6” in diameter