Built my own for off the grid home. A few notes that are relevant to boats too.
Two basic types: continuos and batch. Most boat systems and my DIY are batch. Larger commercial
versions are continuos. With batch you set the contents aside to finish composting. In my case I designed to swap out containers. Set the current
one aside to finish for a few months...on a boat of course not so practical unless you return to the same port regularly.
Agitation. As long as the compost pile is areated agitation is not necessary. Mine has a bottom grate and PVC tubes for aeration. Computer cooling
fan provides air flow and forced ventilation.
. A properly functioning compost system does not emit waste like odor
...just an earthy odor. Mine vents outside so you dont even get the earthy odor.
TP is good for a composting system as is any other compostable material like non meat kitchen scraps.
End result after finishing is rich earth w no sign of original contents (including TP) and no disease pathogens (proven by studies). Safe to use even in your veggie garden.