2010 Roadtrek Agile will not always start.

[Blue Note]

Lately I have been having a starting issue with our 2010 Roadtrek Agile. I wait for the lights to clear, turn the key and nothing. Eventually it will go for no apparent reason. It seems to be a not uncommon problem. Probably not the starter as it would click repeatedly. Battery checks out fine and the rig is plugged in when not in use. I'm going to have the ignition relay checked out, but in the meantime I'm wondering if there is a way to switch to the house batteries for an emergency start. Any help would be appreciated as I have to get on a ferry to get to a Mercedes dealer and I'd hate to get stuck on the ferry holding everybody else up.