I just fitted together a bunch of plastic plumbing parts while standing in the aisle at the store, then bought what fit. It was a couple of years ago, so I don't remember the sizes except that they are standard sizes. The plastic elbow that is attached to the drain fitting in the camper is, I think, a drain elbow for a garbage disposer. That runs into a union of the kind used to connect two pipes together. The black connector attached to the union came with my camper, tucked in the vinyl pouch with the water filter wrench and the appliance manuals. I took it with me to the store to be sure I was getting the right size parts. The clear hose at the end is whatever size fits the barb on the black connector.
If your camper didn't come with one of those black connectors, you may still be able to find something like it at a big box plumbing supply store. I suggest taking the cap off the trailer's drain fitting with you when you go, to help you get the right sizes. The cap is only held onto its plastic lanyard by a screw.
Later: I've been off trying to find a link on the Web to a place that sells the connector I mentioned. I think it would be called something like a "female PVC to hose barb adapter." I can't find one like the one in the pic I posted. I wonder if it would be an item that a camper supply store would carry?
It's never too late to have a happy childhood!
Lee, WU0V, and Courtenay, N0ZDT
2011 Rockwood A128
2000 Silverado 1500 pickup
60W solar system
2000W inverter generator