[BB_TX]

If the lights and the fan are fed from the same fuse it could be a loose/bad connection on that circuit causing a voltage drop when the current increases. When ours was new I checked all the wire connections in both the 120 vac distribution panel and the 12 vdc panel and found a number of wire terminations with loose or barely tight screws on the hots, neutrals, grounds, and 12 vdc positives and grounds.