10-15 gal. is pretty small...the average adult uses the toilet 5x/24 hrs, average flush is about 2 liters, so two people will fill up pretty quickly. If your tank is rectangular, determining it's capacity is pretty easy: in inches multiply LxWxH then divide by 231 (cu.in.per gal). The useable capacity is about 90% of the answer rounded to an even number. It's 90% because a) you're using outside dimensions, and b) that's all the USEABLE space without overflowing into the vent and/or inlet line.
Several people have recommended the Raritan SeaEra
Raritan SeaEra Promo Sheet It would be an excellent choice...Practical Sailor rated it "best budget buy" a few years ago. But if you want a toilet that uses pressurized fresh water instead of sea water, there's much better choice: the Raritan Marine Elegance
Raritan Marine_Elegance Promo.pdf Practical Sailor rated it "best in show" the same review of macerating electric toilets.
You may not be aware that there's only ONE NDZ on whole Chesapeake Bay: Herring Bay. The discharge of treated waste from a USCG certified Type I or II MSD is legal everywhere on the whole Bay...and also on most of the entire east coast south of NC until you get to the FL Keys. So I suggest you also consider adding a Raritan PuraSan to the system
Raritan PuraSan Promo Sheet and also the "Hold 'n' Treat" controls
Raritan HoldnTreat controls (Raritan also offers a
Hold 'n' Treat "bundle" that includes a holding tank, but because you already have a tank, you'd only need the controls.
Take a little time to read all the info at the links...by then my book should have arrived, so spend a little time reading the applicable chapters.. Write down all your questions...and then we can discuss hoses, tank fittings etc.
--Peggie