There are instant heaters used in Class Bs for sure. I think it should probably work fine. Some of the ones I’ve seen have a small pint/quart reservoir, and my guess is they cost barely more than a standard one.
That being said, I don’t really have a problem with a standard model, and it is still fine after 13 years. I think I’d run out of water before I ran out of hot water unless on city hookups with unlimited dump ability.
One more thing from FitRV that talks about the RV models that have them as of about 2 years ago or hybrid models that heat RVs as well.
https://www.thefitrv.com/rv-tips/truma-takes-over-the-world-of-rv-heat-and-hot-water-at-least/
