I first suggest that you take a look at the agreement itself to see if it describes what a breach of the agreement is and whether it sets forth the damages for someone who violates the agreement (i.e., a liquidated damages clause). Replacing the mobile home may have been a violation (if it was not approved by the County Health Department) but its hard to say without reviewing the agreement. Assuming it is a violation, and the agreement does not specify damages, then the party in breach might be held responsible IF that breach can be proven to be the cause of the system failing (that may be difficult to prove).
Because there's a lot of "if's" in that scenario, why not consider having the health department evaluate the situation before you buy to see if the septic system is already overtaxed? If it is overtaxed then perhaps you can ask the violators to help upgrade the system to alleviate the problem before it arises, agree in advance to pay damages if a problem arises, or agree to pay the cost of pumping the septic tank more frequently. If the violators won't work with you then you should consider that carefully before you buy the property. Good luck!
Providing this general response does not create an attorney client relationship.
I agree with Mr. Hawkins and would recommend you have the seller get the system inspected and approved as will be required by the new laws. If the there is a liability question make certain you are insured for this scenario with an additional rider if necessary. Better yet find another house or get the price lowered or remedy the situation before closing.
Only If and until you and I sign an Agreement for Legal Services, I am not your attorney. These answers are provided for informational and/or novelty purposes