Good topic.
What I carry:
socket and wrench set, including screwdriver handle with various size tips
12-volt circuit tester
air compressor
ramp for trailer tire change
jack for trailer (as a back-up to the ramp)
can of fix-a-flat
tire pressure gauge
spare 12-volt bulbs
multimeter
large wrench with wheel lug sockets
soldering iron and solder
fuses, fuses, fuses
phillips and straight screwdrivers
pliers
batteries for smoke/CO detectors and flashlights
wire ties
awning repair tape
That's all I can think of at the moment. I've found that the better prepared I am, the less likely things are to go south.