I have a system down for mine that puts it on complete auto pilot but I only have 2 and it took me a few years to get it right.

There is certainly money to be made by experienced park owners in septic systems, cesspools and wells.

Over the decades, "leaky air systems", bad plumbing, years of moisture causing rotting wood are "termite magnets" for older mobile homes.

Folks with private sewer usually know because they have to pay for the clogs, but the same thing occurs with public systems.

Follow Up Boss is good or your brokerage may also offer a CRM system that integrates internally (be sure, though, that you own the contacts, not the brokerage).  

A plumber (better if they also do hvac) to evaluate the whole system, electrician, hvac.

waste fluids could have been discharged into septic system.(5) you say gas station is currently vacant. have the tanks been properly closed in accordance with temporary closure requirements? 

I got all the personal mortgaging out of my system and then made the switch, but alas I do not have a time machine I could offer you.It is what it is.

 $700 is nothing compared to the costs (Five figures) of repairing entire systems - and with HUD houses you can assume that something is drastically wrong.  

Forgot to mention an entirely new high efficiency combi heating/hot water system with new baseboard throughout as well.