1986 270 sundancer

Hello - Just bought a 1986 270 Sundancer. Has an odd head that has macerator / overboard discharge feature but doesn't seem to have holding tank. Has plastic head / toilet that looks like a porta potty yet plumbed to boat. Want to replace toilet but mechanic not familiar with this setup...

Just bought a 1986 Sundancer 270 Turns out doesn't have a holding tank and seems to have combination porta potty that also can be discharged overboard via macerater. Anyone tried converting to holding tank. Anything in the enclosed fiberglass box area starboard under after berth? Thanks!