IPI "1 Step"
I have been using this stuff for at least the last 15 years. Wire brush the loose rust off and brush or spray this stuff on.... it's the consistency of white milk and instantly etches into the rust and turns black. you can paint over it if you want but don't have to in areas that don't get sun.
When we lived in PA i used to paint half a rusty metal fence post and leave it for years and the painted part rust never came back.
Expensive and i've had the same gallon for all that time..... this stuff spreads very far. I even tried it on rusted quarter panels and it stopped the rust.
Do not remove the rust as it's needed for this product to work correctly
..... Your truck doesn't look all that bad and i highly recommend this stuff over just paint
Rust Converter: 1-Step Rust Killer
I haven't tried this yet with white vinegar but it looks very interesting. skip to about the middle to save all his talking and see the reaction
https://www.youtube.com/watch?v=TlcL_vHODkc