This is a hard problem
When they developed the A-10 Warthog, they started with the enormous, tank-killing cannon. Then they built an airplane around it. If you want a decent toilet in a small travel trailer, it's a lot like that: the water and sewage system dominate the design.
So if your notion is "leave a 24 inch square space for the toilet whatever", you'll need to do some more design.
Human feces is toxic and spreads disease (particularly human disease to an extent animal feces do not). So you must dispose of it properly. There are "dump stations" all over the world to allow RVs to dump their blackwater and graywater tanks. Anywhere there's a canal system, there are dump stations for the boats too. Human waste is never dumped into the environment. That is literally a crime, and you will go to jail and/or fined severely if caught. By "fined" I mean pay the actual cost of a hazmat response. Not cheap.
Everyone - aircraft, trains, military ships, even ocean liners - have holding and retention tanks.
Ask the people doing it
This is widely done in several DIY communities: Tiny House builders, VanLife people making vans into RVs, off-grid communities (who are often borrowing land or using land that won't perc for septic), RVers of course. You could spend many evenings browsing what they have to say in Youtube, podcasts and blogs.
The English have an equivalent: narrowboats on their amazing canal system. They are very economical as primary residences, except for the crushing $200-1000/month for a permanent mooring with hookups. So, many cut the cord and "continuous cruise" the system, tying up anywhere, effectively boondocking 24x7. They have answers too.
The upshot is there are no silver bullet solutions. There are several lead-bullet solutions.
Flush toilets are problematic
A residential flush toilet has two serious problems in a vehicle. Consider how it works in a house. The clean water in the bowl is part of a trap, just like the one under your sink. This "slug" of trapped water keeps sewer gases blocked. This wouldn't work if vacuum or pressure developed in the sewer pipe to suck or blow it out, so the sewer pipe must be vented to the roof to equalize pressure. In a vehicle, the water will splash out of the trap. The vent will smell, and may act as a pitot tube in motion, creating the pressure/vacuum the stack is meant to prevent.
A couple other problems with flush toilets is they use an awful lot of water, which makes your blackwater tanks a lot bigger and/or your endurance a lot shorter. You must also have potable water to flush with, you can't use greywater.
Freezing will make more problems
I have never seen the "freeze it" solution. It seems like you'd have an unresolvable disposal problem. There are many places to do a proper RV cleanout, but all are designed for liquid waste and would not be able to accommodate a frozen brick of poop-water. Pretty hard to dispose of.