Dometic makes much smaller units for the European market.  Coleman used to make a 7700 btu unit that is no longer available.  Now the standard on all units no matter size is a 13.5.  Some micro TTs have window units now.  Trailmanor had window units in a cabinet and they worked very well.
Not sure why we don't have access to smaller units.  The noise and vibration on a small TT will drive you nuts.  In my 18ft Funfinder I quit using the roof a/c and had a 5000 btu window, it ran off of my Honda 2000 overnight, 14 hours on one tank of fuel.  Quiet, effecient and only $79.  
We now have a 24ft with a 13.5, again, overkill.  It's always too cold even in northern Mexico and South Texas heat.  I quit using it except in extreme cases.  Looking to install a window unit where my microwave is now.