Sounds like one of your air ducts might have separated and not allowing the full air flow. Instead, it's blowing under the floor. More than likely, the age of your trailer, the air ducts are that soft aluminum ducts, not the plastic stuff wrapped around a wire spring.
I had a 2006 Springdale, 31 feet long, and it had weak air flow from the beginning, because I'm pretty sure the main line running down the middle of the floor was pinched and squished. I was able to run a very long vacuum hose down the duct and found a spot where it wouldn't pass. I kept working the hose. I think the hose eventually pushed its way through the crumpled part, because after that it started blowing better.
I also learned the junction points were just simply taped to hold them together. So, it's possible yours separated somewhere. The tape finally deteriorated.
I'm assuming it blew stronger in the past?
