3 gallons of any primer is a lot, 3 gallons of single stage color for two trucks isn't all that much. Nothing sticks as well as factory primer and paint, never "strip" a vehicle down unless something is wrong with the paint itself.
I've painted professionally and there is no way I'd do two at the same time, your flash time between coats will be way off and you'll have a ton of overspray. At this point it is what it is, Is the red oxide some kind of old lacquer product? I haven't seen a red oxide product in 20 years!
As said bare metal needs to be covered with an etching type primer, I like Dupont Vera-prime and then a couple coats of decent -good urethane primer. I've used epoxy's before and never again, they sand like crap. Finish all body work with 120 or 150 grit and then one coat of etch primer and when dry follow with a good heavy 2 or 3 coats of the 'fill' primer, let dry over night, then sand with 400, blow off wipe down, tape up, ( do not use the same tape/paper for prime and paint) tack off and spray. Sounds simple but remember there is a reason body shops charge thousands to paint a car. Good luck