At 350K + miles, I'd say a trans rebuild in somewhere in your future. It has already outlasted 90% of them. At the risk of sounding stupid, plan for a rebuild, but drive it until it needs it. This one may go on forever, or tomorrow might be its last day.

Towing may hasten the event, but it sounds to me like you have already been towing with it, so I don't see how owning the trailer as opposed to renting a trailer will make much difference. Just be sure the trailer is in good shape and keep the maintenance up on it as well. Gene