Questions about 80 gallon overflow

[Monterey Diver]

Greetings travelers! I bought a 16 Jay Flight 27RLS with the Elite package. We went on a shakedown cruise with hook ups and then dry. We took fresh water with us and by the time we got to the first site, most of the water was gone. Talking to the dealer folks, we were told that we should not carry our water with us. It seems impractical to do that if we go to a location that is totally dry. One suggestions is to add stop cocks to the ends of the overflow. But the book says specifically not to do that. Are the tanks so flimsy that the back pressure would blow the tanks? Are there any options that you found to deal with this?

Finally on the subject of water, can you recommend a water purifier and possible placement in the system for both city and tank water?

Thanks and Happy Trails!