There's room for more than 9 gallons...but--
There's certainly room for a lot bigger tank than that, but apparently that dealer thinks there's only one possible location for a tank and/or doesn't know of any source for non-rectangular tanks...Ronco Plastics has more than 400 shapes and sizes, over 100 of which are non-rectangular. However, any size tank is gonna eat up valuable storage space...the bigger the tank, the more space you'll lose to it...and you'd need at least 18-20 gallons to gain any appreciable advantage over 5-6 gal portapotty because of the increase in flush water from a manual marine toilet. You'll also be trading a toilet/tank that needs no plumbing and requires no maintenance for a toilet and a tank that require a considerable amount of both. So IMO, your best option on a boat that size is a self-contained system. If all you have now is a 2-3 gallon portapotty that has to be carried off the boat, you can improve on that considerably by replacing it with a 5-6 gal model that can be permanently installed and fitted for pumpout. The only plumbing required is the vent and and pumpout hose. And because it uses so little flush water, it holds about the same numbef of flushes as a 12-15 gallon tank connected to a manual marine toilet.