Regarding whether the chicken tax is due on import, the law doesn't make any distinction between new and used trucks, so technically, yes, you're supposed to pay it.
That said, the tax is levied based on the HTS (Harmonized Tariff Schedule) code assigned to the vehicle on the entry form (7501), which will be completed by the Customs officer. If they're lazy or generous, they'll fill in a 8703 code, which is a general code for vehicles designed to carry people. The tariff on 8703 should be calculated at 2.5%. Your truck should probably have a 8704 HTS code, which is intended for vehicles designed to carry cargo. Tariff on 8704 code is 25% (the dreaded "chicken tax").
When I brought my HDJ80 in from Canada, the Customs officer who helped me used a 8704 code, which in my opinion is incorrect for my car, but since he only charged me 2.5% import duty (which he further rounded down to an even amount), I wasn't feeling like arguing with him about it. It seemed to me that the Customs officer had some discretion regarding how much duty to charge. He was a good guy, he was interested in my truck, and I got the feeling he didn't want to be the bureaucratic a$$hole that reamed me for huge import tax just because he could. Which is all to say it may all come down to luck. If you get a good dude (all I've encountered have been), they'll probably try and help you out.
Regarding how the value of your car is determined, there are several options. I had with me a copy of my Bill of Sale which listed the sale price, but the Customs officer thought that was too high (!) and instead looked up a comparable car in his little book (seemed similar to a KBB type thing). I was OK with the value he determined since it resulted in lower duty for me.
In addition to the Bill of Sale or the Kelly Blue Book (or equivalent), you can simply declare the value of the car. It's no different than when you're arriving from an international flight and Customs at the airport asks you if you have anything to declare. You're attesting to the US government (under penalty of perjury) that the value you've declared is the true value of the goods. Simple as that. Note that if they're suspicious you've intentionally under-valued your goods, they may ask for proof.
Not sure how helpful all that is. Lots of words.