Here's what I did, and the batteries have remained secure for 9000 miles. Had some scrap aluminum square tubing and an old broken ratchet strap. Tubing was just screwed to the floor and the strap pulled tight as I attached the strap - no ratchet involved. The batteries don't need to be held down with any force, just prevented from jumping out of the tube cleats. I've driven on some pretty horrible US roads, and never noticed any sign my batteries tried to jump the fence. Simple, easy, and effective. You could make cleats out of furring strips, or 2x2s. I just had this aluminum left over from an old TV roof antenna. New it might come in handy someday.
When running the cables for the batteries, I did pull down the front of the coroplast just enough to fit feed the cables and hook it with a coat hanger to pull forward to the factory battery cut-off switch. Nothing under there up against the floor. Lots of wires running loose, drooping down and laying on the coroplast. Seems Winnebego wanted to save a few cents needed to secure wires. You can pull down the front enough to see the bottom of the floor, without removing the panel, should you want to be absolutely sure there's nothing under there that screws could penetrate.
The factory used some glue to secure the coroplast to the front cross member of the frame, plus a few screws. I envisioned a time in the not too distant future, when this seal would fail, which would be a great entry point for water as one drove down the road in the rain. I caulked between the coroplast and added aluminum angle sealed and screwed, so there's no way water could enter in the future.
Good luck with your project.
