Hmmm...I do use the trim on the motor when I am loaded, I can not remember if i have used the trim switches on the console when loaded?
The reason I ask is because the trailer trim would not be affected by a faulty trim limiter/sender. So when your using the trim the motor thinks its at the down limit, but on your trailer switch it doesn't have an issue moving all the way down.
Try it out and let us know.
Edit: you could try adjusting the limit before taking it in somewhere, though I don't know how it would of come out of adjustment and probably needs to be replaced/inspected by someone who knows what they are talking about :P -http://www.mercstuff.com/trim_limit.htm