If you don't mind rather a large size, one thing that will clearly work is one of the booster pack/jump start things people sell. Harbor Freight has one that is good enough, and on good sales drops well below $50. From your point of view it is a lead-acid 12V battery of several ampere-hour capacity with a built-in automotive style 12-v accessory outlet and built-in 120V charger circuitry. You could plug it it in at your house before need, carry it to your employer's vehicle, use it for the day, and carry it away.
A bit bigger physically than you might like, but a known solution. My Garmin 855 uses about 125 to 200 mA on the 12V line, and batteries don't like deep discharge, so don't try a solution with much less than a couple of ampere-hour capacity at 12V.