[PTRon]

Long answer.
Print colors on black - I prefer white or silver (but that's just me).
Layout - Phone # on top, second line; website, third line; slogan, fourth line; name, fifth line; as is. Change the font size accordingly. My opinion is that it's more important for prospective customers to know how to contact you as opposed to the business name (at least on work vehicles/trailers). There's nothing more annoying (to me) than speeding down the highway and not being able to read the contact info on a vehicle and when you're parked you want to have as much exposure as you can, parked cars shouldn't block your contact info. But again, that's just me.
Suggestions - Use a different font or color for your name & slogan (both should be the same but different from all the rest of the info), this makes them stand out. Keep the contact info font the way you have it - easy to read. Try playing with italics.
Maybe add a logo, it doesn't need to be elaborate - as a matter of fact; the simpler the better.
Slogan - I like yours.