This is an interesting conundrum. The solution may depend upon how often your large trucks go out to remote sites and you lose communications. Additionally, are you trying to create invoices on the trucks while out in the field?
If the time that the trucks are not able to communicate is infrequent, I would suggest that you install and maintain FA in the home office and use the Sierra Wireless modems/routers to log into the the central office/server. In the event that you can't establish real time connections, simply write down the items that used for that particular job and enter them into the accounting system when connections can be established. To keep the trucks info separate, you could use the "Dimensions" function to create separate locations / ie. trucks.
On the other hand if the lack of communications is very frequent you could run each truck as you suggested with its own FA system. The problem comes down to a simple way of consolidating data. You could export / import the data from each truck into the central office server. [FA has the ability to do this so you will need to research this function.] It will require a lot more work than the scenario above.
Good Luck in finding a usable solution.
Frank