I think I recall with another company I worked for--radio ads did not work for us.
I suggest that your truck should be considered like a moving bill board--almost like free advertising. Put professional lettering on the tailgate, too. Consider a catchy slogan. Such as for instance: "You can pay more--but you can't do better." Also add changeable magnetic signs --suited to the season perhaps. One for crabgrass, one for grubs, one for weeds.
Also be sure to add white letters vinyl decals to your back and front windows. Cheap and you can put up your website address. Naturally--make sure its easy to remember. Be sure local laws allow a few letters in the darkened window tint area at the top of your windshield. Put the vinyl letters on the outside of the glass.