Ask your question here:
Trawler Forum You will get many answers and specific to your region. I am more active there than here since is far ranging, not just trawlers.
As you have figured out, a composting
toilet is good to go. I just tore up my "traditional"
head and installed a composting
toilet (Airhead). You will also see there is a regional area you might hang out in at the
trawler forum for
Pacific Northwest, Coastal BC and
Alaska Panhandle types.
You have lots of choices. I've stopped helping until someone gives me a
budget, then I'll spend time searching for them. But post in the
trawler forum, I'll be looking for you.