Maria, I'm thinking similarly about doing a van conversion--not now, but within 5 years or so as I get older and accumulate more physical limitations.

During previous trips by van I carried no toilet at all--there were bushes, forests, and various forms of toilets in restaurants, lodgings, stores, rest stops, tourist information offices, etc. Those were shorter trips and largely within North America. It gets old, and at times it feels like the whole trip is organized around finding toilet facilities--not the feeling I'm looking for.

I've also used the standard river rafting system--basically, a five-gallon plastic bucket with a liner and a seat. This requires someplace to deposit the liner bags, hopefully responsibly, as necessary. Some people use chemicals with them, some don't. They have the advantage of allowing storage of filled liner bags until you find a place to deposit them (ick), but I'm not too sure about the ecological consequences of leaving behind a trail of plastic bags full of poop.

And I've used little two part camping toilets while living outdoors for long (5 months) periods back in the day--the equivalent of what I presume you're describing. I always ended up with an over-full, un-emptied, impossible-to-deal-with mess at some point--not only is the emptying kind've gross and prone to accidents (use your imagination), but sometimes there really just isn't any place appropriate to deposit five or ten gallons of concentrated poop, whether chemically-laced or not. It's also a bit awkward to walk around with the full bottom section--40-80 lbs/65-130 kg--even when you've found a place to empty it out. I usually ended up going back to digging dispersed cat holes in the woods--the most ecological solution, as far as I know.

Currently I'm thinking along the lines of composting toilets designed for boats and vans. There are a lot of examples, but the first which came up on a search was this one: https://natureshead.net/road/ I've heard some good reports, but haven't actually seen or used one. If they work, even partially, they seem like the best of the available solutions.

I've never traveled with an actual blackwater collection system as you'd have in a large RV or overland truck. If that's what you're looking at I'm afraid the above will be of little use unless you decide to replace it with a composter.

Hope that's somewhat helpful.

Mark