Black Water backing up in shower when driving.

Are you sure it was black water, rather than gray? Shower and sinks are supposed to go to gray tanks, not black.

Only solution that comes to my mind is take shorter showers so the tank is not as full. Or dump more often. Backup into shower when the tank gets too full is a design "feature." Never seen one back up into a sink, but it evidently happened to you.

Flat rubber stopper over drain with a weight on top?

I'm sure. This TC only has one waste tank(38 gallon). Tank wasn't completely full. In fact I would be surprised if it was even 1/2 full on this trip. I only had one TC shower, with limited sink water use. We have taken several trips with this TC and this was the first time it has happened.

I'd probably search out one of the lists of dump stations and use one of those to dump before going home. You'll be surprised at how many dump stations that are out there for free or a nominal price.

All the truck stops and Flying J type stations along the highway have dump stations.

This is only about a 30 mile trip from my house to a small lake in a National Park were I go to camp/kayak. No dump stations available along the way.

Peg Leg wrote:
Flat rubber stopper over drain with a weight on top?

I bought some at Home Depot yesterday:). I really would like a little better fix though.

I bet your tank vent pipe is inserted several inches into the tank, therefore not allowing you use full tank capacity.
Common fix is known as harpooning. If you can see vent pipe via drain pipe, you can attach saw blade to a pole and cut the long pipe short.

Peg Leg wrote:
Flat rubber stopper over drain with a weight on top?

I would go the route of pulling out the plastic water stops that don't really seal well during a "back-up" situation and use black rubber "corks" that are inserted into the top of the drain and forced in. Should hold until you can figure out why it is backing up. I believe the big box hardware stores should have those stoppers in the plumbing section.

Kayteg1 wrote:
I bet your tank vent pipe is inserted several inches into the tank, therefore not allowing you use full tank capacity.
Common fix is known as harpooning. If you can see vent pipe via drain pipe, you can attach saw blade to a pole and cut the long pipe short.

I'll check when we get back this weekend. Thanks.

For now I have the "cork" type for the sink, and the flat type for the shower floor drain. Toilet, bathroom sink, and shower, sit right over the main drain line. I was really surprised it made it all the the bathroom sink. Shower I could kind of understand. It may have had just the wrong amount of fluid in it to cause this problem, because we have used it several times without this issue, and drove with waste water in tank.