Thanks to all of you for your insight and suggestions. This is our first motorhome as such, I'm going through a steep learning curve. In retrospect, avionstream's question is fair and I expect with good intention.
As a postscript, I had my final conversation with the manufacturer's technical department head in Manitoba Canada. After a brief discussion to satisfy his curiosity regarding our two candidates, we agreed on the solution. Specifically, I will install a quarter-turn shut off valve at the overflow tube.
The only potential problem with this is, I have a senior moment and forget to open the valve once I arrive at a destination. There is, according to the manufacturer, "no chance of a pressure build up while driving..."
Tomorrow morning I will install the device, after I go to the vitamin store and buy all their Omega-3 supplements to boost memory. If you don't mind I will report back next week as we are taking a short trip over to the Lake. This entails going up and down many hills, which will be the acid test.