Jon has introduced another possibility. For a less expensive, but totally functional system, locate a small holding tank (a 'pony' blackwater tank) in the location of the OEM tanks...curb-side engine room attached beneath the floor...similar to the MCI OEM holding tank.
Install a standard RV pottie that dumps directly into the small engine room black tank. install an RV outlet macerator on the blackwater 'pony' tank and pump forward into a main black tank.
The 'pony' tank would only need maybe 5 gallons capacity. That's the size of an outboard motor portable fuel tank. You could double the capacity to 10 gallons in order to avoid cycling the macerator pump.
JR