This is not really a legal issue, it’s more a city/county code issue. If thats what the code reads, then thats what it reads. Mobile homes typically dont have the number of family members that a home would which is why there is a difference in the septic issue most times.
It looks like you have 30K sq. ft. of land, how in the world would you fit 4 or 5 houses on this? Maybe a multi unit building would be a better choice.
Anyway, you might be better posting this on the legal forum, but you really are fighting a loosing battle when it comes to code issues IMHO.
Maybe the property is worth more as a 12-unit mobile home park? Could you put newer boxes on it and then either rent out for good income or sell the boxes and rent the spaces, or sell the whole thing.
I have no great love of the county government officials, but fighting them is often a waste of time and money. If you want to argue with them, read over carefully the ordinances related to the zoning for the parcel. See what is allowed.
Then look at the properties next to yours–is there the possibility of buying part for a septic system? Or getting some sort of easement (for $) to allow a septic system there?