If at all possible, I would highly recommend a larger holding tank. One of the biggest reasons that waste lines stink is that way too little fresh water is flushed behind the waste to clear them out and push the nastiness all the way to the tank. That can require a full gallon of water for each flush, although, that highly depends on how far away the tank is.
For this reason I am about to take out my marine head and replace it with the Porta-Potti that I've had unused in the basement for 20 years.
I also only have a 6 gallon holding tank. I spoke to a previous owner who told me he replaced the 12 gallon tank because it was rubbing against the steering cables. When I looked over the installation I did not like what I saw. The holding tank is significantly higher than the toilet, so you've got to pump uphill. If your seals aren't perfect, everything in the hose will gradually come back into the toilet bowl. There's about 6' of 1.5" ID hose, connected to about 2' of what looks to be about 2.5" ID hose. That's a dead volume of over 1 gallon, so you need to flush with at least that much water, and actually more if you realize that the flow is not perfect plug flow. (Sorry for the engineer-speak.) So you get less than 6 flushes from the darned thing, or have to accept pee and whatever else backing up in the toilet bowl.
To make matters worse, my marina's pump-out station malfunctions over 80% of the time. The suction pump is located on short 150' from the suction hose (which is at the gas dock), and 4-10' higher (depending on tide), so it won't prime itself. I told them they need a priming pump at the gas dock to push river water through the empty hose back to the suction pump, but they won't listen.
My Thetford 155s has a 4.3 gal waste tank, but will accommodate about 45 flushes because there's no dead volume to fill. I'll keep the holding tank and hoses in place (plugged, of course) so I can reinstall it if I ever sell the boat.