I won't dispute Ardra's years of experience at all. Those of us who don't boondock, however, where water conservation is very critical tend to take whatever water will spray and use it lovingly. Regrettably, that makes us poor stewards of the environment in doing so.
In the previous coach, we had a six gallon with a mixer feature which effectively raised the hot water capacity to ten gallons, and currently have a sixteen gallon, and can take back to back showers without running out of hot water on either.
It's one of those "your mileage may vary" things, and if we were more responsible with our water usage, we for sure would not have a problem. Twelve gallons seems a fine compromise in our experience.
You will find there is a happy dance that works in the small confines of any RV shower (and they are all small)! and it will be a lot more spirited if you dawdle past the end of the hot water, trust me. Size of the shower will be more your problem, I suspect, than the hot water problem.
If you like having fun using your RV, you will adjust to both; if not, neither will please you. 8)
What little research I did on tankless water heaters for RV's found those solutions very pricey.