probably because MERC is wary that this would draw down the Chassis batteries, possibly causing problems with the vehicle. Since it's MERC's warranty on the chassis, they don't want second-hand warranty issues to deal with.
having said that - I suppose an owner can do whatever modifications they want, if adding a BIRD and a 12v DC continuos coil is your desire. Another limiting factor might be the size and output of the Alternator itself. It may not be designed to handle additional loads as larger powertrains might.
What about a 100w solar panel on the roof? $150-$200 or so, and might be a lot cheaper and easier.
'14 Palazzo 33.3 bunkhouse 34' diesel
KingTailgater2 Dish HD, 100w/5a SOLAR, BlueOx
100,000+mi since '14 - US, Alaska, Canada.... formerly Georgia, then NorthCarolina, now Alabama : )