Neither "customer use" nor "customer usage" is correct. The customers are not being used; the trucks are. I 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.

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.