I considered another valve for the main tank, but since the trailer isn't used year round as a residence, it made more sense to just turn off the tank valve if it ran out and use the disposable.
If I was using the trailer daily, a valve would be handy to allow whatever appliance(s) to operate while the main tank was being filled.
But if that were the case, a dual tank setup with auto regulator would be the choice.
Jack wins the bet...
Rob