A rust conversion/seal before top coating might be prudent.
Ospho Rust Treatment - Since 1947
https://www.magnetpaints.com/underbody.asp
I used:
https://www.eastwood.com/ew-rust-converter-new-formula-aerosol.html (worked it in with chip brush)
https://www.eastwood.com/eastwood-rust-encapsulator-aerosol-red-15-oz.html (didnt work well sprayed and stayed on surface)
https://www.eastwood.com/eastwood-extreme-chassis-black-primer-set.html (no long-term data)
https://www.eastwood.com/eastwood-internal-frame-coating-14oz-aerosol.html (bottle went very fast and covered little but nice tube applicator)
Eastwood was quite expensive $500ish) and I wasn't entirely thrilled with some of them, but there was probably user error involved.
Undercoat the large panels and use the specialty products for the frame and smaller sections.
Use FluidFilm or make your own WaxOyl for the small bits.
A truck will be easy to DYI in good weather after power washing .