PVC is a really bad idea- it's attacked by UV, ozone, heat & vibration. It tends to shatter violently when it fails while pressurized with air. Google "PVC air line exploded." If your tank isn't stainless or aluminum, I wouldn't use it. Steel corrodes & plastic doesn't like UV and high pressure.
A keg tipped sideways and slanted bottom-down like it was mounted on the pioneer rack would work. If 2-3 fit, run them in series for more capacity. As I've pulled my pioneer tool rack I can't go test-fit.
If there's room behind the cab for a angled keg, that'd be another option. Painting the kegs green and adding a canvas water-bucket for a hat to hide the plumbing will make them much more stealthy.
Mental note, get primer and paint for kegs.