Mickey, Some will say that only the line coming from the top of the engine
intake manifold towards the water heater needs to be shut off, and will
support that belief with empirical data and spread sheets. Others will
state that "No, you need two way valves that create a bypass loop back to
the engine". Personal experience with both methods has sent me into the
mind set that both ways will work well. I am firm in my belief, though,
that fewer parts and hoses are better than more stuff. I believe it was
Kettering that said parts left out of vehicles or assemblies have a zero
failure rate, and cause no service problems later.
Jim Hupy
Salem, Or
GMC ROYALE 403

I am at a loss as to why you would want to do such a thing. If you feel the hoses are bad and may fail, replace them.

That said, If you are trying to just turn off flow through the water heater then one valve anywhere in the system will work.

If you are trying to disconnect the one of the hoses and drive the coach then BOTH lines will have to be turned off. You will need two valves or a bypass and shutoff valve connected to both lines upstream. The whole cooling system is pressurized to around 9 PSI while driving and will leak anywhere you open it up.

While you are at it you might as well develop a similar valve system for the cabin heater. It has the same failure exposure.

It seems much simpler to just replace the suspect hoses. I just last night replaced the servicable heater hoses on my 16 year old Blazer. I was in there replacing an intake manifold gasket. The removed hoses had nothing wrong with them after 16 years and could have easily gone much longer.

So if it bothers you that much, simply replace the hoses and worry about it again 15 or 20 years from now. If they are bad, you are going to have to replace them anyway after you turn the valve(s) off.