I carry a little Allen & Heath ZED10 in a small briefcase that lives behind the seat in my truck. it only has 4 XLR's and 3 stereo line ins but it would be enough to get me through most gigs if the DL does poos and wees.
it only has 2 AUX which would be enough for some very basic foldback, and my 4RU rack has a backup DBX 231 EQ just in case, as well as my DBX Driverack PX.
Obviously with so few channels available on the ZED the drums would have to run unmiked for the gig, same with guitar, but i could still DI the bass, keys, or any other DI'd source.