Some trailers and caravans are very long and therefore incur more voltage drop than usual. Measure the voltage drop from the battery to the last electric magnet.
Learn how dirt roads destroy Electric Brakes. Unsuspecting caravan owners discover their electric braking systems and electric drums are being rendered defective by vibrations transferred to braking systems by corrugated roads.
Here is a simple test for your electric drums to test to prove polarisation. Point your trailer coupling facing north or south. Residual magnetism can be excess positive ions(North Pole) or excess negative ions (South Pole).