We have a FunFinder 210WBS. It's 21 feet long with a slide. We have never camped for longer than 2 weeks, but it seems to me it is roomy enough for a longer stay. Reading over what others have posted, I think it is fair to conclude that almost any size trailer will work for long term if you have the right temperment for it.
Many have said that a walk around bed is necessary - I agree. You also said that you might forgo a dinette and use tray tables. I think that is also a good idea. We have done that since our trailer was new. Our floorplan never had a dinette; it came with a folding table and a sofa. I recently made a gateleg table to use instead of the tray tables. It is much more versatile than the tray tables. For some pictures follow this
LINK