After some brainstorming (and a lot of good thoughts from your comments on our sunroom and hot water posts), Mark and I have decided that:

• The sunroom is a lower priority than hot water, and may wait until another year.  Part of this reasoning is based on the fact that Mark actually doesn't consider it a hardship to wash up outside in the winter --- I thought he did.  (Personally, I'm pretty content with our current bathing setup, although I might like to lounge in a real bathtub some days.)  That makes setting up a bathtub and shower much easier since we can just put it on the other side of the wall from the kitchen sink, allowing us to easily plumb directly into the graywater system and to get hot and cold water with minimal fuss.  (As a side note, I also changed my mind about where the potential sunroom should go --- you can see my current thoughts in the diagram at the top of this post.)
  
• The solar hot water system and the backup electric hot water system don't necessarily need to connect together.  Since we use hot water almost entirely in the late afternoon, a batch-type solar hot water system could serve as collector and reservoir during the warm months.  I still haven't decided whether the best option is the trailer's old hot water heater, painted black and placed inside a glass-fronted box (would the insulation keep the tank from soaking up the sun's rays?) or a couple of hoses inside a similar box (or just up on the trailer roof).  Without the complication of a thermosyphon system, though, it definitely feels simple enough to build ourselves.
  
• Our best option for conventional hot water during the winter is probably a 19-gallon tank, which we'd turn on a few minutes before using just like Tom suggested they do in Mexico.  A point-of-use unit could be good, but has two potential problems in our unconventional circumstances --- it would need fancy wiring since it draws so much juice all at once, and it might not work with our low water pressure (we get 1 gpm, or possibly less, depending on how full our tank is).  You'll also notice that I increased the size of the reservoir we're looking at since I forgot that we currently use boiling water to mitigate water temperature, while a hot water heater (even set on high) is only going to raise the temperature of the water to 150 degrees Fahrenheit.  As such, my calculations suggest that we'd need a 19-gallon tank in the winter to raise water from around 40 degrees to 105 degrees to fill a 35 gallon tub.  Presumably, we'll learn to tweak how long we wait between turning on the heater and using the water so that we don't waste energy heating too much water during lower-water-use projects like showering and dishes.
  

Mark tells me we usually see a very slight lull in our workload in late July, so hopefully we'll have time to set up our bathing chamber, hot water heater, and solar hot water system then.  If not, it will definitely be our winter infrastructure-improvement project.  Now I'm off to research whether low-flow faucets and shower heads will work with already very low water pressure....