"The tops of the tables are so unlevel they are not usable...."
Wow. Bummer. I have never had that experience; My layout(s) were built with common 1X4's and ordinary plywood (no L-girders or anything like that) 20 - 25 years ago and I did not run into anything but incidental ups-and-downs. I've only built 2 layouts in my life, though (I don't like doing it).
Your experience sounds like you used really low-grade lumber, but I don't get the impression that you did. I took no special care building mine, beyond common-sense measure twice, cut once. Sympathies. Save the better pieces and go buy some more - checking straightness at the lumber yard.
BTW - I have made 2 major additions to the layout in the last 6 - 7 years - a turntable/roundhouse/engine area and a classification yard across an aisle from it, and the new 1X4's seemed as straight as the "old" ones.
