You said there is an auxiliary tank with it's own regulator plumbed into the propane system. Is that "tank" a tank or a cylinder? If a cylinder, does it connect with a black or green acme nut like the other? Black (low flow) will only service one appliance. Green (medium flow) will service all your trailer appliances simultaneously. Is the regulator a double stage or single stage? Single stage might also effect flow.
If everything you have are cylinders why not just swap the aux cylinder with en empty one and see if things work as they should?