A couple things... the rear brakes have two "air-cans" per drum,the parking (spring brake) is released by air pressure,then the actual "brake can" applies the brakes using air pressure.SMI uses a "tractor protection" valve and is D.O.T. approved ,so that valve could be installed and used with your brake system. I have installed several different brands of towbrakes including M&G,Roadmaster ,SMI, and others. I have moved a coach with the fitting installed but no air-line and didn't notice anything abnormal when braking,so worse case scenario you could apply the brakes enough to drain the air faster than the compressor could supply it (unlikely) ,the spring brake would apply and you would be stuck.Basically the shop is trying to limit their liability ,although the manufacturer of the brake system would technically be responsible .Basically, it will work,but they don't have to install something they feel is not in compliance with DOT rules.
Ken Perry Jr.
2006 Alpenlite 31RL and 03 Chevrolet K2500HD
85 4runner (toy) 91 4runner (daily driver)