I have several 'go bags'. Their contents vary depending on use.
The one I use the most has a spare 1911, holster & magazines. It also has a cleaning kit, spare Surefire light, extra batteries, Glock field knife & trauma first aid kit. This bag does NOT stay in the truck!
I also have a cold weather 'go bag' that has winter stuff in it. Extra clothes, socks, water, power bars, extra jacket & boots. I also have a bunch of Hot Hands warmers in it. This bag is only in the colder half of the years & it stays in the truck for half the year. This is the largest bag.
My summer 'go bag' has same contents as above, but with warm weather clothes.
My summer & winter bags I have used frequently when work unexpectedly keeps me over night away from home.
Another bag I use is for travel when I have a long gun aboard....it's setup for shotgun, Winchester 94 or AR. Contents are related to the long gun that is being carried. I keep extra .30-30 rounds, shot shells, buck shot & AR magazines. Cleaning kit for all three & spare parts for the AR.
The last bag I keep year round has a large first aid kit, 2 surplus blankets, flares, rope, gloves & rappelling harness. (The last item I actually needed one time & DIDN'T have one! A car had gone off an embankment & I had to tie one using rope. If it happens again, I don't need to waste time coming up with a makeshift one.) This bag is in the tool box in the truck all the time.
I split the bags up this way to avoid one HEAVY bag, instead I can grab just what I need & not lug everything at once.