I believe your flooplan looks like this, right?
So, is it the entire back end of the trailer that stays cooler? If so, you need to locate the rear-most duct from your furnace.
First, make sure that this duct is blowing as much hot air as the other ducts in your trailer. If not, then you might have some blockage in the line that you'll need to clean. I would assume that it's similar to cleaning ducts in your home ... you can get tools and such to help at Home Depot and other stores. Other people might have better suggestions on clearing a blockage.
If it's blowing OK, but just not reaching the rear, then it's more of a geometry, air-flow issue. You could do major surgery, tear up your floor, and run custom ducting. Or, you can find external ways to try to direct the air flow where you want it to go ... using panels, dryer ducting, and other means.
But definitely start by mapping out where your ducts are at and ensuring that they're all blowing with equal efficiency.
Good luck.
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