3/4 won't sag over that distance. My old table (built in the late 80's) is stiff functioning, and it's 3/4, and has even spent a few years sitting out in the snow and rain...the rubber matting really protects it. That table also has castors mounted right on the bottom of the table (not the legs!) so that when folded up, it can be a crate dolly. I used to plunk the crate on it, stick all my tack inside, and just roll it into the show building. Always thought I should make the dog pull it in. I also used it often for moving stuff around in the house..filing cabinets and things like that.Hi Deb I like the idea of a ramp at both end you will need a good solid way to lock it in place and have it easily removable to switch ends.I would be thinking of 1 inch plywood and the more plies (layers of wood glued together) the stronger it is .Could get expensive though.But I'd hate to do all that work and have it sag also Thule would feel more at ease the more solid it is.
This link will take you to a picture of the ramp designed for a Booster Bath. Maybe a design something like this???
http://www.petsnap.com/boosterbath.htm
Also wanted to add that in my experience, Newfs who have orthopedic issues are the least likely to be comfortable with newfangled things such as ramps and shiny floors. Others may have a different experience with this...