Nice work on the photo illustrations.
OK, I have two Catalytic Wave Heaters installed in my 3905. A Wave 6 in the garage.
And a Wave 8 in the main living area.
Both plumbed with Quick Disconnects.
As I mention earlier in this thread, my manifold had a plug on one of the 1/2" pipe thread side. It was a bear to get the plug out so I don't advise trying to remove one of the hoses and replace it with a tee. I will explain.
First I put an end wrench on the plug. It would not budge. The I put a 6 point socket and a 3/8" drive ractched. It would not budge. I then put the socket on a breaker bar, said a prayer and pulled. The cheap lag bolts they use all over these units snapped. I had to then remove the bolts with vice grip, I have previous experience do that. Then drill out the holes in both the frame and manifold and install 1/4" bolts with nut. Now I try again to remove the plug. Yeah, it come out but not with out bending the 1/4" bolts holding the manifold in place.
That is why I don't think you can get the hose with the 1/2" pipe fitting loose. I could be wrong.
What I did was run a gas line to a location behind and under my pantry. Doing this was not terribly difficult once I got it figured out. With the wall removed I could determine where to drill. I removed the heater duct in there and pushed it down into the floor allowing me to reach in a feel around for access. Had to be careful as the kitchen grey water tank is right there.
So I connect and 1/2" pipe to 3/8" flare adapter and start my run. The adapter is the one on the right side of the photo.
I first installed another Quick Disconnect at this point for a BBQ. It did not work as low pressure gas would not run my BBQ. So I had to move it. Now the hose is to short so I just use a coupler and made a short hose to fill the gap. The couple you see could be the tee I recommend farther along in this post.
I was going to just bend the hose and go through the floor but ended up thinks that would not work so I used a 90 degree 1/2" pipe elbow and 1/2" black pipe to come through the floor. Oh and not before installing a shut off valve.
Once the pipe was through the floor I installed a 1/2" pipe to 3/8" flare tee.
And plumbed two of them to their locations.
I realize that is not necessary what your are wanting to accomplish but I am sharing my experience it installing these heaters.
So, I looking at your photograph. It appears your heater is sitting on the steps into your bedroom/bathroom area. Is that correct. And if so is that where you want the heater to be locater. Is the heater hooked up at all, and where do you plan to bring the gas line in?
As far as where to tee off of the gas line I see two possible choices. On either of the larger gas line coming into the manifold. Or where the gas line comes down from the large regulator on the passenger side of the truck. I have a Quick Disconnect installed on the high pressure side at that location.
Now getting on to your installation. Obviously you need to get the gas line to the choose heater location. If you put 3/8" flare fitting on each side of the cut hose, you can install and 3/8" flare/flare/flare tee at that point and run your gas line into your trailer. I wanted Quick Disconnects as the heater I have in my living are is moved to the garage when I travel. It stills on the floor in front of the Wave 6 and is strapped piggyback style with both covered.
Covers not shown.
If you decide to tee at the regulator, again be advised that when they install the gas lines at the factory, they must use Lock Tight for thread sealant.
Hope this help some what.