Stay with the self contained system...
'Cuz to replace it with a "real" toilet will mean cutting a hole in the bottom of the boat and installing a thru-hull and seacock for the flush water intake, a whole bunch of plumbing--intake hose, hose from toilet to tank, hose to deck pumpout fitting, vent line--and a toilet and tank that require ongoing maintenance...not to mention losing all that storage space to the tank.A 5-6 gal "pumpable" portapotty needs no flush water plumbing, only a pumpout line and vent...requires almost -0- maintenance...takes up no more space than your current portable version...and holds as many flushes as a 15-18 gal tank connected to a manual marine toilet. Difference in price is significant too...about $200 for everything to install the self contained system va. more than $500 for a decent quality toilet and tank, thru-hull & seacock, plus all the hoses you'd need.