I'm glad you came over here - you are likely to get better responses. However, the responses I gave you on IRV2 are still correct.
Mercedes use of Canbus to control the output to various auxiliary wiring is good engineering but it is not something that use users of American automotive electrical systems are used to. Before I understood how the system worked, I had my dealer check out the system (I had the same complaint you had). Out came a test board with trailer lights attached to it (these are regular incandescent bulb trailer lights), plugged it in to the 7 pin plug, and everything worked as advertised. Why, the Canbus system sensed a load on each circuit and supplied voltage to tha circuit. Your voltmeter and your test light do not supply sufficient load to cause the system to supply the needed voltage.
If you really want to test it, you can use a set of cheap towing lights from Harbor Freight. Make sure they are incandescent bulbs and not LEDs. Plug that in and test your system.
Roger & Mary
2017 Winnebago Navion 24V
2014 Tiffin Phaeton 36GH (Sold)