I just watched a walk-through video on Youtube for that trailer, and I'm not sure there's enough space there for a longer mattress. In fact, it's somewhat tight as it is. If you put a longer mattress there, you wouldn't be able to walk around to that side of the bed.
Of course, watching it in a video, and measuring it in real life can be significantly different, but there certainly is not a surplus of space on that corner by my estimation.
I forgot to mention that we get a little more "bed" because of our topper. We put a 4" thick standard queen sized topper on our short queen, and it gives us a couple inches of "unsupported" mattress length. It's not much, but it might be just enough (it certainly is for us). I'm 5'10", and DW is 5'8". Her feet hang off the end, but she likes to have pillows between her head and the "headboard" (which is the front wall of the trailer). Whereas I sleep with my hair touching the "headboard", and my feet don't hang off.