I found the same problem with our BC 3500RL. I had never come across a unit with this type of set up but according to the dealer we need to have the valve shut off system installed, he thought it would be around $350. I have not checked the BH or Landmarks but mabe they already have this system. Had I realized this unit was this way I would not had purched it.
We just returned from a 8000 mile trip and I can tell you there were many times I could not open all the slides on the MH. I plan too check into the valve system futher.