Agree about the bulkhead/hinge spar as a good way to stiffen up the build. There are a lot of forces in the hinge & hatch areas, so it's good to be able to "spread the load" into as many parts as possible to make a better setup for the hinge. Our trailer is very similar to what you're doing with the benroy design. Our bulkhead is at the spar connection, backing for the interior cabinets & the galley cabinets. It's just 1/4" but helps a lot. It also serves as a mount for the galley inverter & charging module for the solar system.
The galley shelf at the spar has to be used very carefully & checked prior to closing the hatch! We left a small container on the shelf on our last trip (plastic fortunately) and the edge was crushed by the 1/8" hatch interior skin. Might be better to not have it, but we use it all the time & it's handy to have for cell phone charging, odds & ends.
We set the PD4045 panel in the galley & it has access from the back from the cabin interior, through the back of the TV flatscreen panel. Here's a couple of pictures which might help explain things.
This has worked out pretty well for us & there's access if we need it, but so far so good on not needing repairs at this point. Wiring is always a bear & if you have a goodly number of circuits & systems, make sure to plan it out accordingly. I used a small 12 volt battery (AGM alarm panel 1.5 AH type) to test each circuit as I went, just to be sure that each wire, switch, light or circuit was really done properly. There's so much going on with the TV, stereo, speakers in the hatch, running lights, cabin lights, galley 110 & 12 volt, air conditioner, etc. that you don't want anything less than 100% certainty, before you button up the walls & panels.
You might take a look at our build as there's a lot there to help you with your design, since both are so similar.