Neither of the routes in your drawing would be a good choice...the wavy one with dips in it would be a horrible idea, just asking for a clog.
Relocating the tank would be quite a project, but possible.
And worth doing because 6' is about as far as the typical manual or electric toilet can move bowl contents with a reasonable amount of flush water without a LOT of help from gravity. It's not necessary for the loop--which doesn't have to be vented if you're only flushing into a tank--to be a mile high--your toilet can lift up to 4', but that's about all. .. just a few inches higher than the top of the tank, then down all the way to the tank inlet fitting. And also immediately after the toilet (elbow discharge fitting aimed straight up), so that you only have to flush long enough to push the flush over the top of it...gravity will do the rest. That the waste will be macerated only means that the salt and sea water mineral buildup will be mixed with waste particles.
How big is the tank? 'Cuz if "it has run like this for 30 years and it must have worked,"
that may be true only because the previous owners filled the tank with more flush water than waste trying to keep the toilet discharge hose rinsed out enough to prevent clogs...and you have no way of knowing how successful they actually were in doing that.So it's entirely possible that a smaller tank in a location that makes sense (a LOT closer to the toilet!) would hold even more actual flushes...not to mention make plumbing it a lot simpler requiring a LOT less pipe. Ronco Plastics makes top quality 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. Ronco Plastics marine Tanks
And btw, although hard PVC is less expensive and odor permeation-proof, it's still just as susceptible to sea water mineral buildup as hose, and also requires a bunch of unions that not only complicate installation, but also trap bits of waste with the minerals. I'd go with Raritan SaniFlex hose Raritan Saniflex Sanitation Hose
(Defender has it for about $10/ft), which has also proven to be 100% odor permeation resistant and has the added advantage of being so flexible it can be bent like a hairpin without kinking...which makes re-piping a LOT easier!