We tow a 4,600 lbs Hummer which has an "active braking" system and can't use an under the hood system. We use a Roadmaster 9000 which is a shock absorber looking thing that fits between the brake pedal and a bracket under the seat. It connects to an air fitting that is fed by the coach's air brakes. It is reasonably easy to install and remove, as it can't be left in place when the car is driven. It works well.
Several friends have, or had, one of the "box" systems that fit between the seat and the brake pedal. Most of them have had problems, either with the car brakes being worn out or car batteries run down, or the system not working when it should. Two other friends have the M&B system that fits under the hood permanently and they haven't had any problems. (I couldn't use that M&B system.)
