Well that sucks. Did you clear coat that truck? The side doesn't look very shiny. If you didn't, that could be one reason why it was trying to get chalky.
No clear, just straight out of the gun.
My Willy’s I painted with Summit single stage and is still shiny.
If you have air, spend a few extra pennies for at least automotive paint.
I think when I painted my Willy’s a gallon of red was only like $140, plus thinner and reducer.
No clear needed.