decaturbob...
Caution! Drilling holes in your chassis frame (or welding) is not a good idea unless you get it right.
If you drill the holes in the horizontal flanges of the chassis, that will weaken the chassis. Those flanges carry the bulk of the stress for load carrying. If you do drill holes put them as close to the center of the vertical part of the chassis rail as possible. That carries the least stress and a hole there will have a minimal effect on load carrying. If possible consider making the unit clamp onto the frame rail. I installed a commercial track bar on my chassis and they made it install with a clamping method.
Also a thought on your concept: if you intend to make the shelf hinge down to access the tanks and boxes, consider the weight you will have to lower and lift, especially if one of the tanks is full of water. You might want to make more than one so you can lower sections at a time. If you have the bottom of the added unit extend past the frame and extend to the outside (or end) of the RV, you would have better leverage to lift and lower and that would provide a more gradual ramp for loading and unloading.
Only you will know what you can do when you look at your situation. Be ready to revise your plan to suit.
RTsRV