U4L,
I've been servicing elevators for over 25 years. When it comes to tool bags, boxes or pouches, I prefer a pouch with a belt to carry on my shoulder. In my pouch I carry the essentials;
A fluke multi meter
3/8, 7/16, 1/2 & 9/16 wrenches
Nut drive set
Screw drivers, P2 Philops 1/4 standard long & stubby, 4" - 1/8 standard
Small Chanel locks
Needle nose plier
Side cutters
Wire strippers
Small snap ring pliers
Contact burnishing eraser & file
Rubber door wedge
Assorted door keys
Two AA LED flash lights
Pencil & sharpie
Assorted fuses
4 jumpers (that must be counted before each call and before the elevator is placed back into service!!!!)

And my personal favorite a 26 year old to carry the tools and run steps when I need something from the truck. Or I'm feeling mean!

As for Maintenance I keep a second bag for a grease gun, Nylube, rags and ketchup bottles (for oil)...