George, a couple of thoughts.
Head to YouTube and check out Alliance factory tour video’s to get an idea of tank to frame relationships.
Get an endoscope(30.00) that you could place a small hole in the coroplast and snake it up in the area you want to drill through.
Remove a few underbelly nails in the area and reach up in there to see what you can feel on the rail.
I have even lowered a small section on the opposite side and looked across to see if the area was cleared.
At the end of the day, I have installed various p clamps, exhaust hangers and never even gotten anywhere close to my tanks using a reasonable length fastener.
You could even predrill using a drill stop set to the thickness of your rail, this would control your depth and prevent that surge at breakthrough when the bit exits the material.
If you do a little local research on your rig, you will reduce the risk and increase the reward.
Proceed with caution.