It only requires finding room for a tank that, on a houseboat is likely to be in the 50+ gallon range if it's to be close to the size of the holding tank...it'll have to be vented, and a new deckfill installed 'cuz toilet flush water tanks
cannot have any common plumbing
with the fresh water plumbing
, not even vents or fills...rerouting the intake plumbing... Shouldn't cost more than $500...
Methinks a gauge for $100 makes a LOT more sense. If the tank isn't metal, you can use one which has a sender that goes on the outside of the tank, so doesn't even require any holes.