Hi Catfan...IMO replacing the tank with a larger one would be a last resort..but if it comes down to that, Ronco Plastics Ronco Plastics marine Tanks
would be your best source.
Maybe all you need to do is use less flush water. How to do that depends on the make/model age of your toilet and whether that's doable with it, or whether a new toilet may be the answer (and make any sense).
The Raritan Hold 'n' Treat system is another possiblity...legal in your waters and almost all water south of you. Since you already have a holding tank, the treatment device--I recommend the Raritan PuraSan instead of their ElectroScan--Raritan PuraSan Promo Sheet
and the controls holdntreat controls
are all you'd need.
So...what is the make/model/approx age of your toilet? Sea water flush or pressurized fresh water? If pressurized fresh water, you'd be able to replace the flush button with one that has 3 options--bring water in ahead of use, dry flush and full flush. It could increase the number of flushes your tank can hold by up to 50%.
"If you can't explain it to a six year old, you don't completely understand it yourself."