ya personally it seems like it's very inefficient, heat = wasted energy.
The original on our 98 georgetown used a CCI detector, it had a 9v selenoid and I think I read that it actually dropped down to 5v after it was open.. iirc
in any event it ran much much cooler.
the 12v relay is probably a cost saving thing since you already have 12v on the electrical system.. the detector can just use a simple relay to cut off power.. doesn't really "manage" the solenoid beyond that.
your's maybe different though we replaced the now defunct CCI with a safe-t-alert / solenoid combo.
anyway one of the reasons I put a switch in was the old cci had a switch but it also killed the detector, it was both on or both off.
the safe-t-alert model we put in had NO switch, so it and the selenoid are ALWAYS on.
so what I did was put in a 5amp (that's what the detector called for) panel mount fuse holder, If I yank the fuse the detector and selenoid go down, but we can still keep power from the house batteries.
then on the solenoid line coming from the detector i put in a lighted switch.
so it's very easy to tell if it it's on or not.
the detector is still in control of the solenoid but I can turn the solenoid off without disturbing the detector.
when we get propane or pull into a station we can just turn the gas off with the switch inside, don't need to touch the mechanical valve on the tank outside.
we also turn off water heater and fridge just to be on the safe side, they can't get any propane cause the solenoid is off but they might still spark trying to light, but still it's a lot easier and can all be done from inside.
battery -> fuse -> detector -> switch -> selenoid