It would seem that the first question you have to answer is whether or not the tank farm would be even noticeable. For a thing to have an effect on value, it will have to rise above the minimum level of perception of a significant number of buyers. Unless prospective buyers are saying "the tank farm is a PITA, I dont want live near there" seems like there would be no negative price effect.
It sounds like you've determined that the primary impact would be traffic related. Have you eliminated terrorist threat, blast radius damage, fire/smoke, environmental factors associated with leaks/spills to the air, surface water, or groundwater, visual impacts, etc etc? I've was told by an appraisers that works in the bay area, that around the refinery there's a difference in pricing between up-wind vs down-wind because of the occasional fires and releases they have. A tank farm is not a refinery, but a similar effect relating to the potential for danger might be at work.
If its just traffic that you're worried about you need to get a handle on what their work schedule would be.
100 trucks a day is 12.5/hour based on an 8 hour day.
6.25/hr on a 16 hour day.
4/hr on a 24 hour day.
Big difference. You also need to find out if 100/day is 100 drive in's and drive-out's (IOW 200 passess/day).
But for any of those numbers to have a negative impact they would need to represent a significant (noticeable) increase in traffic. 200 a day is noting on an interstate or major thoroughfare in an industrial area, but its a killer on a less traveled road. Presumably an environmental impact report exists for this project, that should discus traffic and might have some good resources for you in the bibliography. You might also check with the county to see if they've done a traffic survey on the roadway which would speak to the magnitude of the existing traffic. Worst come to worse, you might have to pop out there and count trucks for a 15 minutes at stretch at different times and days of the week to document what the baseline traffic is like. If you can document that the tank farm traffic is a small fraction of the total existing traffic it should make it pretty easy to substantiate a "no negative value influence" conclusion. If the additional traffic is significant you'll have a good basis saying there is an effect at which point you're only arguing about how much.