My 05 Tropical was the same, no battery hold downs on the coach batteries. I used a 3/4" steel angle iron ans two threaded rods to make my own hold down. The angle iron lays across the top outer edge of the batteries and the the threaded rod is mounted two places thru holes I drilled in the angle iron and the bottom inner tray. The rods are at 45 degree angles down to the rear of the trya and provide full support. Coated the whole thing with POR15 paint treatment.
2005 National Tropical w/Cat 350