While neither "customer use" nor "customer usage" is incorrect, I suggest an alternative.  My reason for doing so is that the customers of your trucks are not being used; rather, the trucks are.  I therefore suggest the following:

>I am currently studying the patterns of truck usage by our customers.

Or

>I am developing a profile of how the customers who buy our trucks use them.

You can then add the following:

>We are interested in variables such as distances driven, fuel consumption, the number of years they keep their trucks, and the number of our trucks they have in their fleets.