this may vary from state to state, I have an F450 and I need to have dot number, and inspection, and a class A when I pull my trailer since the weight rating is over 26,000 in combination, but as far as the lettering goes just becouse it is lettered does not make it a comercial vehicle, I see trucks all the time with fishing this and that and racing this and that on their trucks this does not make them commercial, unless you are paying them to do this, look up the dot law's in your state, (highway patrol) look up their web site it will be explained there if you do not see what you are looking for call them, new laws are added all the time and old ones change frequently..
//chopper