First off, after listening to the numbers being thrown around I am realizing that there is entirely too much fiber on our boat!
As others have said as long as you disable the ability to discharge in some way you should be good. They make marine
all in one units. If they are full-ish they are not fun. I have never used a composting toilet but hear nice things.
As to urine being sterile. That is a little more fuzzy. It does have bacteria that is normally found in the bladder. If there are any... problems they will be in there as well. I don't think drinking it is a good idea, even in a survival situation. I am not sure what the TDS levels are in urine, but I am guessing that they are in the same order of magnitude as seawater. Especially if you are dehydrated. I know that some cultures use drinking of urine for various reasons. I am just saying do a little more research
before that becomes your go to plan.