For the record, that flat black is most likely a Primer, and you really want to paint it soon, as Primers don't really hold up all that well.
You don't really want to use Rustoleum. Get an automotive enamel and it'll hold up WAY better. Different paint has different uses.
What's your painting ability?
Do you have a compressor, and a cup gun? If so your best bet is a quality automotive paint, reduced and sprayed. Two or three nice coats will be fine for a work truck.
Something like this is what you need.
Not a Painter? No Biggie. With a little patience you can get good results from a rattle can paint job. Go to an autoparts store and get a couple cans of something like the
Duplicolor that matches your truck. Take the tailgate off the truck, clean it well, wipe it down with the appropriate thiner and spray about 5 or 6 thin coats from your rattle can. Read and
follow the instructions on the can for recoat and dry times, as well as fan overlap. Take your time and it'll come out just fine. Probably about $30-$35 worth of paint and an afternoon. Most people that mess up spray paint use too heavy a coat or uneven coats.
A lot of really good paint is the skill of the painter, but for an 11yo work truck, you should be able to put down an adequate finish, that'll look at least as good as those Maaco hack jobs with no real problem.