Keeping computer powered up during start cycle

Hello,
I am currently having a problem with computers in fire apparatus and ambulances. The computer the city has decided to go with is the Intel NUC, which requires 19V input. I started out with a simply 12 to 19 volt converter and the voltage was dropping enough during the start cycle to cause the computer to shut off. I called Lind Electronics, who makes power supplies for computers and explained the problem. From them I got a 12 to 19 volt power supply that was supposed to remain stable down to 10 volts for 10 seconds. I'm still having problems even with these power supplies. Also, sometimes the monitor and the mobile router (Cradle Point) will blink out during a start cycle, which causes additional problems.
I am looking for a simple robust solution to keep a solid 12volts to these components during the high demand start of a fire apparatus.
I have one with unit a backup battery fed off a relay that disconnects the whole shebang while the starter is engaged, but it is having similar problems also. Doesn't seem logical but it's happening.
I need Simple, I really don't want a bunch of other stuff to cram behind panels or in consoles.
I need Bulletproof enough to survive in a fire engine.

Ideas and suggestions?
Thanks
KJ

Get a couple of these and mount them into a metal bar to use as a heat sink. Put this array in series with the charge wire that charges the battery for the system. No need for a relay to disconnect during start cycle. If this works you may want to use 4 per vehicle. If a battery goes completely dead it may take more than 50 amps depending on charger being used.

https://www.amazon.com/NTE-Electronics-NTE5829-Silicon-Rectifier/dp/B008UTVRO6/ref=sr_1_3?ie=UTF8&qid=1504314453&sr=8-3&keywords=50+amp+diode