The forum has plenty of threads about transmission oil coolers, the advisability of installing extra ones, and reading the transmission temp.
Those threads are based on earlier models (not VS30) but have info that probably applies.
In short, transmission temps are held very closely to the engine coolant temps by the cooler built into the radiator. A few folks with T1N based motorhomes have seen higher temps on hard uphill hauls, but transmission failure via overheat does not seem to be a widespread problem, at least not by number of reports on the forum.
A few folks have hardwired temp gauges in their rigs. However, he easiest way in monitor transmission fluid temps has been using an X Gauge on a Scan Gauge II - you can find the entries here
https://www.scangauge.com/x-gauge-commands/2019-mercedes-sprinter-3-0l-diesel/
The forum has not had lots of info on the use of Scan Gauge II in VS30 and there has been some reports that it doesn't work - so no guarantees if you go this route. But if it does work, you can monitor your transmission fluid temps and decide for yourself the need to put in extra cooling.