The ECO and an adjustable thermostat for the Atwood, I was able to find online at marksrvparts. I used thermopaste between the water tank and thermostats to get a true reading for the thermostats(the thermopaste transfers heat)I had a problem with oxidation in this area because the tank is alumium and the thermostats are steel. I got the thermopaste and the neophene pipe insulation(sticky on one side)(to cover the thermostat area on the water heater)from grainger.com.
You can't just turn up the adjustable thermostat to gain more hot water,you'll burn yourself.
I installed a Honeywell adjustable automatic thermostatic mixing valve (for about $100)on the back side of my water heater.(which I also got from grainger.com) With the adjustable thermostat set on high and super hot water,this mixing valve automaticily adds cold water to a set point that I adjusted to 115 degrees as my outlet temperture. It took me a couple of water heater cycles to dial this in and I don't have to redo this adjustment.I have a 6 gallon water heater Atwood,Now I have 9 or 10 gallons of hot water.
I had a good water heater,a larger water heater would not fit in my trailer,I wouldn't spend the money on a on-demand water heater,and if I can modify it or fix it myself to save a buck I'll do it.