I did a similar bypass job on the road for our 1995 30’ cutter The “bypass” worked well until we could get to a dealer for repair.
Our leak was in the heater core up front but the technique should be the same.
The “junction box just under the hood on the upper left” that you found is actually the heater/air conditioner unit. That box contains the heating and cooling “cores” and the fan for those systems. That is where we had our leak, in the heater core.
Because the problem we had was the heater core, I inserted the bypass assembly up under the hood to bypass the entire heating system. If you do this you will have no heat when you are driving. The “rear heat exchanger hose … near the side door”, that failed on your rig is either the supply or return hose for the rear heater (usually under the step) and the water heater. I think you should be able to put your bypass under the rig near the bad hose.
I made a bypass using plumbing parts we got at a little old country hardware store in Turbeville SC.
Here’s what I did.
I used pipe elbows and fittings because a piece of hose bent in a “U” for a bypass would kink and the water could not flow. The parts where two 90 degree elbows, one 2” pipe nipple, two pipe to hose adapters .and two hose clamps to clamp the supply and return hoses to the pipe to hose adapters.
I took a piece of the hose to the hardware store to make sure I got the correct size parts.
I hope you find this helpful and good luck with the project.
BTY... It sounds like we may be in the same business. Are you in the film or TV production business?