I have a 2000 E150 GTRV with a porta-potti. There's a guy in the UK who is trying to launch a compost head that is the same size. He's looking for a go-fund-me style initial interest of $20 to hold a deposit against a 25% discount for launch in the US.
I don't know the guy from Adam, but I like compost heads (and hate chemical toilets), and $20 is cheap enough to risk losing it. I just received an email that states his 25% discount ends Sunday. Post-discount price will be $500, and it seems like a nice head. Believe it or not, $500 is a bargain for a desiccating head.
For those unfamiliar with compost heads (really, "desiccating head" is a more accurate description), here is a decent YouTube video describing how they work. I'm familiar with them due to a cruising/boat background - those that have them love them.