In a previous life, I worked for 3M, and their automotive division supplied many industries, ranging from railroad passenger cars to airplanes, with reflective striping and lettering - lettering could even be ordered with a paper paint mask on it so the lettering could be applied, the vehicle painted, then the mask removed. Doing it that way sealed the edges of the lettering so it wouldn't be affected by wind and weather.
Ambulances can be lettered with vinyl graphics films, or with Scotchlite reflective striping. The reflective striping can easily be a DIY project, but the reflective material is definitely not cheap.
Ambulance reflective striping is usually applied in 4-6" widths, and is often floated on by spraying a slightly soapy solution on, then squeegeeing the stripe into place with a plastic tool. Joints are mitered with a razor blade. After it has thoroughly dried, a layer of clear coat can be sprayed on it to protect it from the environment.
If it's properly applied (and you aren't parking it in Las Vegas in the sun), the life expectancy should be 10-15 years or longer.