Yep, finished now! The view from underneath:

The rubber elbow looks great in there and if it was house flexing or ice in the elbow that cracked it, the rubber one will be a big improvement.
View from above:

The rubber elbow is under the elbow going down on the right. The pipe assembly wanders along the floor, up the side of the door, then back to the sink trap and was an ungainly monster before I put the rubber joint in. I took the opportunity to remove the section connecting to the two sinks and including the trap for cleaning. When I first ran water down the drain, there was a big leak in the cupboard from the trap joint to the drain pipe. There was dirt in the joint. It was sure nice to unhook the rubber joint and take both pieces to the shop to blow them clean.
I just remembered someone mentioning supporting the rubber joints - I must remember to put something under the horizontal run along the floor to support both of them. Thank you for that!
Before I closed off the hole in the floor with white plastic sheeting, I noticed a considerable flow of cool air coming up through it. Hot day, door and vent open - and the hole in the floor was more cooling than the door. I wonder if anyone has seen an RV with a floor vent to take advantage of the cool shade underneath and the natural flow of hot air up and out the roof.