I need to replace 2/3 of the current water heaters.

Even accounting for electric, water, and sewer hookups needing to be set up, it seems plausible that buying one of these as a rental could offer a better ROI than constructing a freestanding ADU building.

For instance, if you have 50 residential units, with 50 roofs, 150 toilets, 50 hot water tanks, etc. you have a huge liability at any given moment where nearly all of the repair and maintenance expense is on you.

Expect to at least pay for water/sewer.Where is your tax number from?

If not, I pass.The first time you get a $600 water bill for one month from a single unit because the tenant "forgot to tell me about the leaking toilet" will help you understand why. 

If you leave them exposed they will serve as entries for water. 

What is the lot rent on this park, and who pays for the water & sewer cost?

spent around 10K doing some cosmetic updates and new water heater. got it rented two days after a quick renovation.

Your going to need proper sewerage electrical and water setup for that to be legal and get any funding .

For example, often fire damage is water damage--they put the red stuff out with wet stuff.