Looking for a Porta Potty recommendation

Thetford is well-regarded. That's what we have and it's fine. Ours isn't quite the cheapest they offer but close. Seals up very well (enough to swell with pressure going over high passes) and since we have little kids, it gets used. There are tricks to getting a clean flush as well as avoiding the dreaded "potty blowback".

If you can sit on them before buying, it would probably be good. And you probably should not allow standing...