Wow! And I tough I had messy set up in my trailer
I see water regulator and safety valve behind the black pipe. I see the problem with access there. Without having 3-D pictures I can only guess the whole setup, but I would start with taking the black pipe out. You can cut off in convenient places and than use rubber couplers to reconnect it.
The water regulator has union on it. The big nut on left end needs to be turned ccw. Put one big wrench on regulator body in horizontal position and other on the nut going down at some angle. If you place them right, you can squeeze 2 wrenches by hands only and break the nut loose. Doing it this way will avoid potential knuckle damage. After disconnecting the union you should gain some slack to bring the corner flare fitting and disconnect it there.
Since safety valves are not required and yours is probably frozen like mine was, I would get rid of it. This way using hose regulators you might gain that space for carrying spare parts.
BTW flare fittings lost the popularity lately to compression fittings. I am planning to use compression fittings on my plumbing. I am good in soldering, but think what if you get into plumbing problem while camping? With some spare compression fittings you can rearrange whole plumbing with just couple of crescent wrenches and tubing cutter.
On edit I can't tell for sure the regulator union is on the left end. Take a close look and see which end has bigger hex. This is going to be union.