About 6' is the max distance that most toilets can move bowl contents in the amount of time anyone is likely to spend pumping a manual toilet or leave their finger on a flush button, so waste will always be left sitting somewhere in any toilet discharge line longer than about 6' that doesn't run downhill all the way to the tank. So install a second tank. Boat builders install only one because it costs them less. You can use a smaller tank on the toilet that gets the least use. Ronco Plastics Ronco Plastics marine Tanks
(no relation to the VegoMatic Ronco) is your best source for a tank. They make TOP quality thick-walled water and waste tanks for a very reasonable price and have more than 400 shapes and sizes, over 100 of which are non-rectangular, and they install fittings in the sizes and locations specified by the customer when they make the tank.
Using hard PVC instead of hose in a system that has a lot of bends requires installing radius fittings, each one of which becomes a potential leak...so use it only on long straight runs. Although hose that really is odor permeation proof is a lot more expensive than hard pvc, it's a lot easier to work with. Raritan SaniFlex RaritanSaniFlex hose
is the only hose I recommend because it is "odor proof" and is also so flexible it can be bent like a hairpin without kinking. Defender has it for <$10/ft and WM price matches if shipping cost is an issue.
Replacing hoses is a lot easier if you connect old hose to new and pull it through as you pull the old hose out. Use a male-male "hose mender" fitting...cut the ends of both hose hoses cleanly to create the smoothest unbroken surface as possible when you butt them together. Use PVC cement or any "glue" that'll keep the hoses on the fitting...duct tape won't hold, clamps can get caught. Cut the new hose off right behind the fitting.
You'll find a lot more useful info in my book...see link in my signature, just click on the title (my publisher's idea), which is actually a bit misleading...'cuz although it does deal with every source of odor on a boat and how to cure, or better yet PREVENT 'em, it's actually a comprehensive "marine toilets and sanitation systems 101" manual that explains the laws, describes all the types of systems and how they work, and will help you learn how to operate and maintain your system to prevent 99% of problems instead of having to cure 'em. 'Cuz you get to do any preventive maintenance on your terms when it's convenient...the need to cure a problem never happens when it is! And I'm always glad to answer any questions it doesn't.
"If you can't explain it to a six year old, you don't completely understand it yourself."