I think its up to the RV manufacture if they what a "CHMSL" (AKA - 3rd brake light) when its over a certain Gross Vehicle weight.
I dont have one on my 2017 Axis but I put a cheapo LED Receiver hitch light from Harbor Freight.
I figured I would eventally install a high up brake light but I kinda like the low mount in the hitch (for now).
For one it was easy, Two since most RV's are taller than the cars and SUV's the Receiver hitch is not too low. Three my Mtn bike might block any brake light that is higher.
So I leave my slide in LED light hooked up all the then then when Im hauling my bikes I have a slide in receiver mount bike rack ....then I take two zip ties and install this hitch light below it so its still operational and I think it looks petty darn good. I dont have any pictures.
2017 Axis / 25.4: Big Foot Levelers, Aims 1200 Watt inverter, Ceramic Window Tint, Full River AGM 6V batteries, Front Hellwig Sway bar / Rear Track Arm, Safe T Plus Steering Stabilizer, PD 4655 converter, Hard Wired Surge Guard, WEboost + WiFi Ranger, LED Headlights/Driving lights, TPMS, Surge tank, Tornado flush, Viair Compressor