If there is room at the foot of the bed and it doesn't interfere with anything, the mattress can be replaced with a longer mattress. You may have to so some modification to support the longer mattress but in many cases, it can be done.
There is
NO room to have a longer mattress in a 181. That said, I'll let my wife tell you about how the length is, since she is right at 6' and I'm quite a bit shorter. From my viewpoint, you may or may not be comfortable in a trailer this small. Unfortunately, the mpg line is so new that there aren't many used ones available, so if you can't rent one for a weekend you may end up buying one just to try it out. The good news, though, is that there seems to be a demand for these little trailers, so if you do buy one and find out that it is too small you ought to be able to recover a good portion of your initial investment when you trade for a larger rig.
We just got our 181 in December, and have spent only a few nights in it, so we're still getting used to it. You will definitely want to put a memory foam topper on the mattress, though. The first night in the trailer was a bit "rough" since we were just in sleeping bags on the mattress. As soon as we got it home we put a topper on the mattress. The second night (with the topper) was
much more comfortable!