Well, good you had the tank put in. Discharging waste (black water) overboard
is illegal, with good reason, in any US inland waters (Federal law).
Re grey water, in most places it is not illegal to discharge grey water directly over board (unless restricted by local laws). So, you could probably just replumb to discharge grey water overboard
. If you want to go to a different tank then I would go with rotomolded polyethylene (same thing they likely, hopefully?, used for your waste holding tank) these are impervious to just about anything and have no seams to fail.
You could also go with a small sump with an automatic pump. Like one of these Shower Sump Pump Systems
. These work out pretty well, but you do have to periodically clean the screen
on the pump.