Sales based upon special equipment use/rental helps track if purchase of equipment was justified and if further purchases are suggested.  Create a structured code eg 3 trailers each 20 foot.

code TR20A, TR20B, TR20C  if you want to track each trainers use or just TR20 to just track over all trailer rental.   

Use this code on each invoice for the service provided.  You can add the codes in reference file 15 - rate or charge codes.

When creating the invoice just type TR and all the codes will come up for you to select the code you want to use.

You can check activity by running Reports \ Financial \ Sales \ Report 37 - charge code to get listing of sales for each charge code with activity in the seleced time period