why do you want to have both computers running?
wouldn't the laptop be able to do everything that the win98 machine could do? and vide versa?
Or why not just hook up a screen to the desktop machine?
Im not really familiar with the inside of a cab, but i would figure you would be able to hook up a screen.
I'm not sure you would be able to run the machine 24/7 how many amp hours do your batteries hold? what other things would you have in the computer, ie hard drive,dvd,gps, etc?