The disadvantage you have Combat1 using an immersion element is that the water is possibly warmer at the top of the tank than it is at the bottom. Even if driving mixes that water up, the warm water will rise to the top of the tank and you will draw off cooler water.
I can measure my hot water as it exits the diesel heater. I have regulated my mixer valve so the water temp is around 51 degrees C at the heater. At the rear van port that water is hot to the touch. At the brush head that temperature changes from reasonably warm to luke warm depending on the outside temperature, which hose reel I'm using (one reel has microbore and the other minibore) and the amount of hose out. It also changes with what the hose is laying on, ie. grass, stone or paving.
The only thing you can do is measure the temperature of the water going into your hose reel and compare it to the temperature of the water exiting your brush head. That differential will be the same with a diesel or gas heater. What I'm saying is that if your water is 30 degrees entering the hose reel and 20 degrees at the brush head, those figures will be the same with any other form of heating at that moment in time. We also find that to begin with, it takes a while to heat the hose up, so the water gets warmer the longer we run the pump for.
With diesel or gas heating, we are just heating the water we are using - heating on demand, so that's the main benefit. You are heating a full tank of water, but you may only be using 1/2 of it on a particular day. It reminds me of night storage heaters. You have to switch them on the previous night in expectation of the following day being cold.