Has anyone replaced a hot water tank with an instant hot?

Tankless heater doesn't seem practical for the application. Whether your overall goal is conserving water (not just hot water) or weight, how much could it possibly save?

Plus sounds like you don't even use or need a water heater based on your post. Why lighten your wallet?
Idk how resilient they are to freezing either. Regular water heater, drain it, get most of the water out and you're fine.

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/

We talks a lot about RVs being cheaply build, but I admire quality of the appliances that are supplied for them.
Have 2 TC, both 18 years old and everything works just fine. WH are with aluminium tanks and beside 1 circuit board I had to replace couple years ago, I have no complains.
The old refrigerators are good as well. Just did another test a week ago restarting Dometic for vacations. Started fridge late afternoon on 120V mode and after 80F night, in the morning empty freezer had -10F.
Not bad I say.
The tankless Webasto heaters are on the market for at least 30 years. Why don't they become popular in all diesel vehicles?