Bypass valve should be right on the back of the hot water tank.
You should have some kind of access point to it from the inside.
If you are unsure where it is or can't find it, locate it by the access cover outside your rig, then look in the corresponding place inside your rig.
Open the door, take off the access cover or whatever is there to access it. Mine is in the back corner and I have to take the mattress off the lower bunk and lift a section of the plywood bunk base to see the HWT.
Look for a pipe going from the top to the bottom of the tank with a valve handle in the middle. They are usually marked with a label of some sort.
Turn it 90 degrees.
Let us know if you get this figured out.
I seriously doubt there is a blockage in a pipe.
Don't give up!