The county, Chester County will oversee the installation of any NEW system.
I have always seen septic inspection reports require the tank be emptied AFTER the septic inspection as a requirement of the "satisfactory" inspection. It is NEVER a good idea to have the tank emptied before the inspection.
Always include an inspection of the septic system as part of your purchase agreement.
However, the standard agreement of sale used by Realtors who are members of the PA Association of Realtors has a septic inspection contingency that includes language that can require the seller to empty the system as part of the inspection. That contingency is an option in the agreement that the buyer can (and should always) elect and the seller agree.
If you make an offer on a property with on-site septic be sure that you elect the septic inspection contingency. Septic system failure can be very expensive to cure.
Rob Hughes- Long and Foster RE.