The original table was 5/8" thick MDF, 30" wide and 42" long with an 8-1/2 x 8-1/2" triangle cut out of the two forward-facing corners to allow people to slide through more easily. Lynne and I -- who are slender by today's standards -- felt the opening was too narrow to allow us to get up and down readily, so we shortened our table to 37" and that seems to maintain table space and allow comfortable access in and out of the dinette.
Good choice, moving to furniture-grade plywood over MDF. Not only is plywood stronger than the standard Scamp
trailer table, it's much, much lighter. To maximize your weight
savings I'd suggest making the table out of 1/2" plywood and build the edges and attachment points up with 1/8" thick solid hardwood, then your table would be the correct thickness, strong enough to sleep on, and much, much lighter than the "stock" Scamps table.
Another approach would be to buy 5/8" A-B plywood and apply plastic laminate (e.g. Formica) to the top and edges. Still lightweight, but easier to keep clean.