Welcome new skoolie friend.
Composting toilets were discussed in high detail a while back and found to be a wast of time.
Use a 5 gallon pail, line it with a garbage bag, do your do, cover it with wood chips, peat moss, kitty litter, ect, and put the lid on.
Or you can use grocery bags as single use containers. same procedure, but tie it up when done. From there you can let them build up in the pail, or take them out after each use.
I incinerate mine after each use in winter in my coal stove. Coal burns so hot there is no smell, and every pathogen and germ is completely killed by the intense heat.
Around here places like Canadian Tire sell 5 gallon pails for $5 and the seat to fit is $10.
Most county's allow up to 10% human waste in regular garbage disposal. After all that's where baby and adult diapers go.
If you still want to compost, still use the pail, and do the composting outside.
It's just not realistic to do it inside the living space.
So yes, a 5 gallon pail will fit in a shower pan without trouble. Simply remove to use the shower.
"Don't argue with stupid people. They will just drag you down to their level, and beat you up with experience."
Patently waiting for the apocalypses to level the playing field in this physiological game of life commonly known as Civilization