Right. It's not that the sway controller will be significantly less effective. Its that the strong forces from the sway controller could cause the front wheels (or trailer wheels) to lose traction when turning.
I think the main point is that if you find yourself in the rain, use extra caution when turning. Keep things slow so that your turning wheels maintain traction (no understeering into oncoming traffic!), and the camper wheels don't skid sideways.
thebrakeman ('70), DW ('71), DD ('99), DD ('01), DD ('05)
2004 Surveyor SV261T (UltraLite Bunkhouse Hybrid)
2006 Mercury Mountaineer V8 AWD Premier
Equal-i-zer WDH (10k), Prodigy Brake Controller