Did you get your water heater all up and going?
There is no actual water shut off to it. There may be what they call a by pass valve setup that bypasses the heater and in a sense, it does shut off the water to the heater by directing the water around the heater. The white water valve on the bottom of the heater has to be turned to allow water to get into the heater. It is directly on the back of the heater. The white lever is normally horizontal to let water into the heater, and vertical to bypass the heater
The back of your 2003 Sunline, should have a water heater that looks close to this.
Your 2003 Sunline would of come with a combo gas and electric water heater. Meaning it can heat with LP gas or 120 VAC electric. Make extra sure the water heater has water in it before turning on the electric element. It will burn out in seconds with no water in the heater. If the heater is bypassed, water can still come out of the hot faucets, just not be in the heater.
The 2003 campers, when you want to heat with electric, you have to have the circuit breaker on in the power converter box for the heater and the little switch on located on the back of the water heater, then it will start heating by electric. Many folks have such a hard time getting to the little switch on the back of the heater, as it can be buried under a bed or cabinet that they "might" have added their own on off switch out in the open. That added switch would not be in any manual.
For the LP gas part, by 2003 there should be a tank sensor panel with a red rocker switch on it that says HTR for heater on LP gas. The water pump may have a red switch there too. It might look like this.
Sunline did change features every now and then to the next upgraded model, so if what I am saying is not exactly like yours, let us know or post pics and we can help you figure it out. 2003/2004 models was a design change year on the Atwood water heaters. You might have one or the other water heaters, but I am fairly sure the one above in the pics is the one you have.
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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