I agree Navy showers leave something to be desired, although we resort to them quite often as we like hitting dry camping spots. I'm also not much of a fan of on demand water heaters, having spent 9 years overseas while in the military. Temperature control tends to be primarily flow control which is often inconsistent and results in a somewhat uncomfortable shower.
It sounds like you already have a new TT. How big is the hot water heater on it? Does it have both electric and gas capability? As others have mentioned, the newer quick recovery tanks can keep the hot water running quite awhile, especially if you end up using something like an oxegenics shower head, which has always been one of my first upgrades with a new rig. We just purchased a new 5er which has a 12 gallon hot water tank. We haven't used it much, but so far the wife and I have both been able to take showers as long as we wanted, without turning the water off to soap up. Just a hot refreshing normal shower. 10 or 12 gallon hot water tanks seem to be available on many rigs now.