Didn't realize this until after the tenant moved in and texted "tubs not draining, toilets, not flushing, sinks aren't draining"...

Just treating the bad unit will flush the nasties to safe harbor in the units that have not been treated and then you will have your good tenants mad at you.Since those nasty tenants have already done their damage but are still willing to live there...you might as well collect as much rent as you can before you decide to force them out.  

For example,I install the standard ball valves underneath every fixture/sink, use TOTO Drake toilets (strongest basic flush system ever!!

Many of our landlord clients have clauses that make the tenant responsible for drain backups if they flush something down the drains that shouldn’t be flushed.

You turn off the water, pull the tower off the wall, unscrew the leads, flush the system, connect the new tower, and hang it back on the same brackets. 

I had a vertical crack in my basement, all along the height of the wall, and it had started to lean in on one side, so the crack was no longer flush.

Renovation financing of existing structures to meet current guidelines will likely be the better bet while this all gets flushed out.

While lots goes into the analysis for a Mobile Home Park (for which I am less versed - due diligence will flush those issues out) this much I do know. 

take care of the water heater (flush it, change the anode rod) and it will be good for years.  

If it's a clogged toilet, pictures won't help and are completely unnecessary: either the toilet flushes or it doesn't.If an owner asked to inspect their property, I try to accommodate them but I require them to give reasonable notice to my staff so that we have time to schedule it with our office and the tenant.