Yes, I see what you mean on the diode and they are leaving something out as far as I can see.
I did some snips of the drawing and what I see as the way it works. I used green for the start battery that I know is found on the "isolated stud" they show, goes to the left side stud as drawn to meet a wire going to the chassis breakers and when the mode solenoid is pulled, it goes through the solenoid to meet the coach battery I drew in blue. The coach battery also goes to the battery disconnect switch and if operated, on to the breakers for the switched coach items.
I agree that I don't understand the diode on a line which goes to ground in two ways ( Wire FM is shown as ground and it also goes to the mounting screw for ground!) By normal rules on drawings, the current only passes in the direction of the arrow, so they are showing us current would pass from ground to meet the battery at that single stud on the solenoid.
Connecting the incoming battery from the boost switch directly to ground without the diode would be expected to blow a fuse!!! But they show the current going from ground to battery???? And nothing goes into the solenoid coil to make it operate? They "could" drain AC current off the ground doing this but why there is AC on the ground would make me wonder!!!
So we are both confused? So what happens if you use a different source to pull the solenoid and that source is only on when the key is on?
One problem I might see is that it does away with using the boost switch to "jump start" the RV engine.
So if you want to chase it a bit further, I see LR as going to a pin 7 of a plug in second right compartment and then on to sheet 2 and feeding a "battery changeover switch" and that involves the "Ford run only circuit"????
Do you know of the changeover switch? Before doing away with the boost switch feature and not sure what exactly, I might want to check this stuff a little further.
If you are not getting battery on LR when the engine is running or when you push the boost switch, I might go with the idea that there is a problem along this route.
OR I might just be pulling your leg to see if you really like crawling all over the RV!!