You may not be able to fit a larger tank into the space your 6 gal tank now occupies, but that's not the only place to put a tank...nor do you have to be limited to the off-the-shelf rectangular tanks
in the marine
catalogs. Ronco Plastics Ronco Plastics Marine Catalog
(no relation to the VegoMatic Ronco) makes TOP quality thick-walled water
and waste tanks
for a very reasonable price
and has more than 400 shapes and sizes, over 100 of which are non-rectangular (which means you actually might be able to fit a larger one in the same space), and they install fittings in the sizes and locations specified by the customer when they make the tank. So before just assuming that 6 gallons is all the holding capacity you can have, check 'em out! They're great to work with.