I'm in Australia so the environment is slightly harsher and the distances are longer, but here is what I know from the feedback of my customers:
1. The temperature varies from one side of the vehicle to the other, especially on long journeys.
2. The loading has an impact. It effects air flow and the overall thermal mass.
3. Temperature inside the container is not uniform.
I worked with one company that used about 50 units at once to do a total profile. It helped demonstrate beyond doubt the shortcomings in the transport method they were testing. That's overkill if it isn't for published research.
The minimalist approach is as you said, one in the truck and one in with the goods. Following, however, is my recommendation:
Within the truck, place them at various locations (front, back, each side). If you have a uniform temperature distribution within the truck then it doesn't matter where your container for testing is located. If you don't have a uniform temperature profile then you obviously need to test the warmest location. Understanding the temperature profile within the truck first is possibly more meaningful. It lets you know the temperatures you are working with.
Depending upon the type of vehicle and its colour, the temperature inside will be significantly different to ambient temperature, especially when it is stationary for a period.
Go with a couple in the goods - top, middle, bottom and one side. You would expect the middle and bottom to remain cold the longest.
In terms of type of logger, there are some really small, low cost units on the market now. The thermocron is possibly the smaller temperature logger on the market and so you can easily fit it anywhere within your goods.
A multichannel offers the benefit of simultaneous results but the cabling is a nightmare. They now tend to cost as much or more than multiple temperature loggers.
I hope this has been helpful.
This post has been edited by Simon: 19 January 2012 - 10:17 PM
Reason for edit: Removed promotional link as per TOS