Where are the manufacturer's in this? Sounds like you're doing a fine chunk of work on their behalf, saving them the time, money and inconvenience that goes with customer support, and making them a profit at the same time.
Is it possible to defer certain calls directly to the manufactures?
Is it possible to pressurise manufacturers into giving you some support for effectively doing their job for them? Might they, for example, be affable to the idea of offsetting part or all of the costs associated with taking on an employee (or employees) specifically to deal with queries about their products?
Alternatively, could they be persuaded to help offset 'training costs' or indeed any other costs that might help you run the sales support aspect of your business more effectively? It would certainly be in the manufacturers best interests to help you support your, and their, customers.