From memory, the bypass at the water heater on both my Roadtrek and my B+ looked pretty much the same but there are no doubt lots of diferences between RV's. The principle remains the same though.
Basically there are 3 valves and two short pieces of pipe added to my B+.
The cold water in line has a "T" junction then has a valve after that just before it connects to the water heater.
The hot water line has the same. The two "T's" are connected together and the third valve is in the center of that line.
You shut off the two valves on the cold in and hot out lines and open the single valve on the two connected "T's".
The following photo from
shows it better than I can explain it:
From the photo, you turn off the valve labeled "outlet valve" and turn off the valve labeled "inlet valve" and open the valve labeled "bypass valve" to bypass your water heater.
Lot's of great info at http://www.ccis.com/home/mnemeth/plumbing/plumb.htm