We have a cheapo Subaru and just use those collapsible silicon buckets, a sous vide, and a cheapo Amazon pump+shower combo. Entire setup is less than $120 and now we have a sous vide too.
You can run the heater off a battery bank (we have an eco flow with the panels). Much easier
Also, towing and road trips are like the number one reason I think most outdoorsy people should be going for plugin hybrids for now. You can be 100% clean (local utility dependent, limited to 'fuel' and not other maintenance) for like 95%+ of your trips but still have that extra oomph if you need it