Okay, I have an idea gestating here...I'm thinking the GoGirl with a long tube attached, like four feet long or so; drill a hole through the floor in the front of the TD to act as a sleeve, (it'd come up next to the mattress) with a tube through and a bucket underneath the TD. I took my TD on a week long trip to a music festival, and the hike to the portajohns became an issue. The Go-Girl could be tucked down under the cabinetry and pulled out as needed, keeping the tube running through the sleeve in the floor.
I'd just have to figure out a way to seal up the hole during transit so if towing in the rain, etc...water tightness...don't want to rot the wood or end up with a soaked mattress.