Form 440, MC property tax q's

I am a trucker's wife, who doesn't work, except that I am a housewife, mother of 2 boys, homeschooler, and I do all the paper part of the business.
I have come to realize I may be doing the form 440 wrong... the MC Property Tax form. We bought an older Freightliner and car hauler trailer together. On the form first sent to me it only had the tractor listed, not the trailer. I assumed they still wanted property tax on both pieces of equipment, so I put the total we paid for both of them down and figured it from there.
Now I'm wondering if I should have listed the trailer, too, or if I don't have to pay the tax on it at all (only in the beginning when we first tagged it), or if I am actually doing it right. I'm sure they want their pound of flesh from it, so I have a hard time believing I don't have to pay tax on it every year.
I'm just not sure if I'm royally screwing up or not.
I hope this isn't confusing... if it is please let me know and I'll try to clarify.
Thanks for any and all help!!! All of the forms and taxes confuse me a little, esp when they don't send instructions with them that are worth a darn!

Oh, and I live in SC, if that makes a difference.
Thanks again!