Wow, nobody answered this!
Voltage drop depends on cable length and amount of load. With a 6 sq mm cable over the 6m from front of car to rear plus say another 6m to the axles of the van seeing a voltage drop like that when the load comes on isn't actually a surprise. What is a surprise is that some installers will use 4mm cable - it's rated at 20A, sure, and your brakes will draw (for 10" drums) 3.3A each so that's 13.2A across two axles - within the rating of the cable, but unfortunately turns the cable into a heating element. And yes, it gets hot enough to melt the insulation - and then the caravan has no brakes at all.
So, step 1: check ALL of the cable. Car and van. If either is thin, that's your culprit right there.
Step 2: make sure the 12-pin connector (you ARE using 12pin, with a viable earth return?) has good connections on both sides. I rewired mine and brought an earth on 21 sqmm cable to the rear and connected it to the plug (yes, I had to strip a little of the wire away to fit it in there). The earth return is important because it makes the electrical circuit complete.
Step 3: Grab your DMM and measure the amps going into each magnet. A standard 10" drum brake magnet should pull about 3.3A give or take a little. If it's drawing more, it might be faulty - and the more it draws, the larger the voltage drop.
