Along with the bulbs themselves, try plugging the umbilical cord in to the truck outlet, and unplugging it several times. It is common for the connections there to become dirty or corroded and just 4 or 5 in and outs of the plug in the outlet will wipe the contacts clean and they will work again. I often have to do this when my trailers and tow vehicle have been sitting for a few months. Sometimes only one thing will not work, like the running lights, or one stop/directional light, or even the brakes themselves, and everything else will be working. That causes some head scratching.
So, try that along with your other things.