The phrase "battle bus" usually refers to a vehicle used in video games, specifically in battle royale games, where players gather and start the competition. Full definition
The allegations centre around the declaration of spending on Conservative battle bus tour in 2015, which took activists to dozens of marginal seats before the election. (theguardian.com)
I have every reason to suppose that the arrangements that we had and all parties had with national battle buses, they get reported in the required way. (theguardian.com)
Publisher biteback Biteback said the book would cover the battle for Brexit, from the «massing of the UKIP foot soldiers after the general election and the squabbling of the various Leave groups, to the arrival of the cabinet cavalry after Cameron's Brussels deal and the dramatic final weeks fought on battle buses and in TV studios». (totalpolitics.com)