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

What do you seek to improve? The current water heater is there and works just fine as-is, right? At no extra cost.

Instantaneous heaters usually need a short time to reach the desired output heat - some extra water is probably wasted that way.

Usually, fresh water is a more limited resource when traveling than LPG.

You would also reduce your fresh water reserve by the 6 gal. in the boiler tank.

In winterizing it might save a few minutes.