when we bought our rv, the guy had an SMI brake system on his towed veh. Said he would remove and give to us. Well he did and i installed it on a new hyundai elantra. It is not visible in any location as the unit goes under the hood, the system taps into the vacuum system at the brake booster and there is a small activation cylinder on the brake pedal arm. It is hooked to an on/off switch which is in the vehicle and out of sight. We towed this summer about 4500 miles and the only issue was that I did not have a charge line on the line for the hyundai. About 2 hrs later we had one and no issues after that. It does take some gtting used to to avoid the cylinder on the pedal but depending on the veh it could be a non issue. The hyundai is fairly cramped for the controlls so it can have more room for the passengers!
Anyhow from what I have seen it is competitively priced with all the others, does not require anything more than hooking up the light cable and putting a switch in the on position. No getting in out to install/remove anything then trying to have astorage issue for the component when not in use!