If the truck will be outside, you need the primer to actually seal out the elements. Many primers are porous and do not seal out moisture. The rustoleum would likely be ok if you were topcoating in that paint session so the finish would be sealed. As you described, spraying rustoleum primer only is going to mean stripping and re-priming down the road if your truck is sitting outdoors. The epoxy is your only choice in this case, IMO.
