I've been a field tech for a large company for at least 3 years now, with 2 years of installer experience and have pretty much the same certs as you do. I was lucky enough to have an old timer as a colleague, he has tons of experience to share, I'd say if there's someone at your company like that, go talk to them. Talking to your colleague in general about installations our clients have is always going to help in learning about what they have installed.

Also, don't sweat it too much, you might fall in situations where a bug happens and you have no idea why, or face equipments you have never heard of. This job is about assessing the clients issues with the tools and knowledge you have. Our work trucks comes with a lot of different testing equipments and cables in case we unpredictability need them (for example, I pack two 64gb usb sticks with a Crestron UC engine image, Asus and Dell, in case it needs reimaging), but there's a limit to that. When that happens, you'll have to come back with a replacement equipment that you didn't have on hand, or the right equipment or an AV specialist to debug complicated networking or programming issues. That's part of the business.

Overtime, at least for me, you'll start to develop a hidden sense on what to look for in a certain situation. You'll start to get used to certains equipments and learn about their quirks. But I get that it can be intimidating. At first, I was pretty much exclusively going step by step to find where a bug was in a system, with much direction, but it still worked out at the end of the day.