105F max? That's hardly hot water in my book, closer to warm...but I guess that's neither here nor there.
You can still get manual pilot RV water heaters, and they have a manual adjustable thermostat similar to a household water heater. Another option might be to install a thermostatic mixing valve (sometimes called a tempering valve) at the water heater output and set it for whatever temperature you deem appropriate.
I don't think there's too much of an energy use difference in the grand scheme of things. Not too much heat is radiated to the outside world from the water heater even at 140 degrees or so. That said, a lower temperature will be somewhat more efficient, as the rate of heat transfer is proportional to the difference in temperature (in this case between the water temperature and the ambient temperature), and heat thus transferred is "lost" and must be resupplied by the burner or element.
