There are many options on the market from composting toilets to flushable camp ones. I use a bucket with a toilet seat that I bought from Walmart for $25 that does the job. I line it with single-use grocery bags and sprinkle cat litter at the bottom of each bag to mask the smell. If you tie each one off and dispose of the bags when you take the trash out, there is no perceptible smell. You can empty your trash in big dumpsters. That way, you’re not throwing poo bags directly in small public trash cans, because no one wants to smell that.
It’s important to separate your solids and liquids (meaning pee and poop) to keep the mess to a minimum. You think those bags you poo in are reliable and can hold liquid pretty well, but there always seems to be a leak somewhere. What I think is a game changer for car living is a pee jug, and everyone has their favorite type. I recommend using a big clear jug so you can see how full it is and to avoid overflow. Sure, you can see the pee through the jug, but it’s better than going pee one night, not realizing your jug is full, and spilling all over the floor. The best pee jug I’ve found is the Arizona Ice Tea or Mucho Mango jug—it’s clear and comes with a hefty handle so you wont drop it. To account for the small hole, I use a pee funnel, or SheWee, which allows me to stand and not worry about aiming. Other less ideal options for females are squatting over a wide mouth pretzel jug or nalgene.
Another thing to consider is your gray water, or waste water, that comes from your sink, if you have one. The best option is to collect gray water in a jug. The bigger the jug the less you’ll have to empty it, but the longer dirty water sits, the more it smells, so keep that in mind. I use a 5-gallon jug and haven’t had any issues regarding overflow or smell. I recommend finding a clear or light-colored jug so you can see the liquid level, but I have a dark blue jug and shine a light to check liquid level, which works fine as well.