If you're set on electric water heating, it could help things immensely if you consider a high voltage battery bank -- to the point that the heater can be run directly from the batteries without any inverter, for example. This is very much into the realm of engineering it yourself..
As an example, consider a battery bank at 96+ V dc paired with the Eemax SP3512 point-of-use instant electric heater. It's not too far-fetched to think that heater could be modified relatively easily to run from a dc source. The Home Depot product page
indicates it'll give 35F temperature rise at 0.7 gpm, which isn't much, but thought experiments have to start somewhere. Anyway, 3.5kW from 120 Vrms is 29 amps so it looks something like a 4.11 ohm resistor. Put that across a 96 V battery bank and you'll draw 23.4 amps or 2.2kW (note that already we're getting less temperature rise and/or water flow than advertised). But 23 amps from a battery bank is nothing. Losses in the batteries and wiring would be kept relatively low, and most of the power would be delivered to your (luke-warm) water. A 5 minute shower would consume 0.186 kWh, and if you add 15% for losses (probably too optimistic) that's 0.214 kWh. Rounding up a little more, that's about 1 of maybe 5 daily hours of useful sunlight falling onto a single 235 watt panel.
From my perspective that's a very meager supply of warm water, but maybe it's enough for your purposes. At this scale it seems doable.
One other note: if heating water or the interior space is a big thing for you, it might be time well spent to look at using the sun to heat the water directly. Storing energy via heated water isn't so different from storing electricity, and if ultimately heat is the energy form you want anyway, you can gain some efficiency by not converting to and from electricity.