The names in my makergn example are irrelevant. You just have to know whats input and output. Only the region matters.
If you run makergn without any arguments, you will see:
I am surprised that you can write .rgn made with .img file directly into a region and it works. Nuvi's are different from 2720 in that region 48 is actually a dos type disk image with a partition table that has files inside of it. Think of Nuvi region 48 like a flash drive or a hard drive.
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By now it should be clear I have zero experience with 2720 type Garmins.
If you want to study what sendmap/updater sends to gps, install usblyzer/usbtrace and start logging. Wish you luck.
I don't know how or if its even possible to dump regions with 2720. Maybe someone else with more experience can IDA the firmware to see the commands the device will respond too.