The typical tenant in affordable housing doesn't have the knowledge or financial means to maintain these systems properly (if it 'aint broke, don't fix it), and as a result the Park owner is the most suitable person to do it and pass those expenses along to the tenant - or otherwise risk a major capital expense hoping the tenant does it right, especially for those systems that cannot be replaced like for like due to stricter environmental regulations.
For Parks not passing along these costs today with private utilities (at least in Texas, probably other states) this means registering as a public utility (25 page application + 6 month review cycle) where you propose the various services you shall provide and get agreement on the rate, which encapsulates well service / maintenance, EPA reporting, septic pumping & maintenance, average break / fix scenarios, etc. over the course of a year. Calculate this into a PUC agreed blended monthly rate per gallon you can pass to the consumer. There are notification legalities, etc, but is all up front process establishment as part of the application, and not very complicated for most MHP investors.
The original poster asked about a 1:1 septic to home ratio - I think it would complement the above mentioned billing processes well as you can state in the lease that any repairs due to tenant negligence (e.g. personal wipes, grease, etc) clogging the system shall be a repair they are responsible to pay. This would further reduce unforeseen expenses and have a higher likelihood of maintaining a static rate for private services, only adjusting as service providers increase prices for inflation or other factors every few years.