Marine toilets have no holding tank at the toilet. They pump directly from the bowl to the holding tank. Adding a tank at the toilet would necessitate a second pump.
I never found that my holding tank in the boat filled very rapidly, but we just pumped the bowl empty after each use and then pumped the line empty when we were ready to dump the tank. The approximately 8' of 1 1/2" line holds quite a bit in itself. I remember that our carry tank was about 8gallons. It was heavy enough to lift out from under the cockpit seat that I woould rig a block on the boom.
John W. Irwin
2005 Classic 28 "Sabre-Dog III"
2013 Silverado 2500HD Duramax/Allison LTZ
WBCCI #9632, TAC TX-10