We have just started a field inventory program using phones as inventory scanners. Everyone uses the same inventory sheet; distributed and shared from a cloud-based Google account. The app is free, Google Drive and Play have all of the apps you need to get started. Inventory is on a google sheet. Yes, there is some work to do to set it all up and automate it, but my Youtube account is my modern public library card.
We have established service contracts with our larger accounts and we tend to only sell certain types of equipment and devices (NVR's, cameras, motion devices, access readers, door contacts, hard drives and the like). We keep the parts list as small as possible by using an established group of vendors and manufacturers. We offer our customers an "On Board" inventory guarantee. If you choose us as your vendor and use our recommended equipment, we keep the top 50 most frequently used items on board each vehicle to maximize your uptime. No coming back to a site to replace. The top most frequently used/replaced/consumed parts are what is on the trucks for service. The inventory sheets are reviewed daily at 3pm and inventory replenishment goes out that day on UPS. Install jobs are staged and delivered to the site as an entirely separate entity.
All of the parts are scanned by barcode using cell cameras which populate a Google Sheet which the field personnel have access to with WIFI and, of course, the office has access to with their accounts. WIFI even in the warehouse. Everyone uses the same (shared) sheet which is updated, real-time, all day.
The thread already mentioned is an excellent source of what tools and equipment you might want each vehicle to have on board (ladders, laptops, cable, connectors and hardware).
Greg's comments are pretty much how the real world works. Tech's work hard but paperwork is not their forte'. Good record keeping is absolutely vital and can save your corporate keester. Always be on the lookout for ways to automate the paperwork.
Our way may sound like overkill, but let me share a general real life application.
If you remove an NVR from a site for whatever reason, that NVR has hard drives in it. You have potential evidence of a crime on your service vehicle bopping down the highway. You must be able to prove chain of custody of that evidence in a court of law. Good record keeping is gristle. Real-time access is gravy.
When we evaluate a potential vendor/partner one of the many criteria is whether or not they are willing to share their barcode database. It has their codes, parts descriptions and our prices. Inventory is automated and accurate.
Everything mentioned here can also be done with paper and pencil. Just think it through and start somewhere.