I need to chime in- 12 years as a "pro" bedliner shop, started with. Line x for the first 4 years. I've seen and tested nearly every incarnation of product intended for the job, as well as the well thought out, and not so well thought out ideas to use other stuff.
For your dump body, I agree with the previous comments about sliding. Don't do anything that will grab. From a rust protection standpoint, a coating only works of it is completely intact. We've all seen the rusting scratch on our fender... So any coating will only help until it's used the first time in a dumptruck.
For your septic truck- don't do anything that will form a membrane, like ANY kind of truck bed coating! Whether rolled on or sprayed on. Even the cheap ones will create a membrane that is inherently strong within itself. What I mean is it will make a "sheet" that exhibits high tensile strength and doesn't tear or break easily. What ends up happening is that since the coating doesn't chip, crack, or flake, any failure (especially inside a tank) will end up as a complete failure. There is no "feather it out and touch it up". You will have a "bag" within the tank. At the very least, you will wind up with huge sheets of loose failed coating plugging up the works!!
My recommendation for both, would be Penetrol. It's a paint additive, but works wonders on bare steel as a rust prevetative. Cheap, easy to recoat anytime, doesn't effect future ability to paint...