I have a 2017 MB Sprinter chassis with a 2018 Thor Siesta mh. I used to have a 2016 Winnebago ERA on a Sprinter Chassis before. Each time I had chassis battery issues. If the chassis battery gets too low or dead, the ignition key will not turn. I then bought a small solar battery tender system for about $25.00. Each time I store the mh, I plug in the solar battery tender under the hood to the red battery jumper and the ground. I then run the wires through the front driver's door and place the solar panel in the windshield locking the door before leaving. I also had my dealer install an emergency house battery to chassis battery switch so I can actually jump start it from the house batteries. I never have a chassis battery issue now. I just have to push a dash button to activate the connection.
The emergency jump from house to chassis battery, sounds interesting. I have had no issues with mine, and I do always carry a couple of chargers, so I could always get started, even if I had to run the generator for 120 volts to plug the charger into, if one of my inverter powered outlets wouldn't be enough. Batteries are always a PITA.
Bob- You mention coach battery and battery disconnect- both are not part of the chassis electrical. If your rig has been stored, for some time, the Chassis battery may be dead. It is not charged by electrical power (120 volt power cord) connected to the coach, in most Sprinter RV's.
Check your chassis voltage here (Red Connector) and also connect a charger at this terminal - driver side of the air filter. I use a 'Batter Minder' when not in use for a long time. Plug in to power point (cigarette lighter) in the lower dash..
I have a 2018 Sprinter Citation. I've heard talk of an "emergency start switch" for starting the chassis from coach power. I don't see one. Is this installed on my unit?