We always carry a "Luggable Loo" ($19.99 on Amazon) - it's the old 5-gallon bucket, but has a nice toilet seat and lid that snap onto the top. It's basically the same height as a toilet. When lined with a trash bag and some kitty litter added, it makes tossing the contents into the trash a simple operation. Lightweight, too, and doesn't take up much space. Also provides a 5-gallon bucket in case you need to haul water, etc.
Luggable LooI should add that if privacy is needed, setting the potty on the ground next to the vehicle with a door open, and a gentleman holding a blanket on the other side, should offer enough coverage that a lady in need doesn't feel overexposed?