Hello & welcome.
If it is like our prior Class C or current Class A motorhomes, that fuse box is a chassis manufacturer item. (Ford E450 & Ford F53 in our rigs.) So, search the interwebs for a fuse box diagram (or even better - owner's manual) for your rig's chassis/model year.
https://www.gmcmi.com/wp-content/uploads/2014/05/P30-Chassis-Manual.pdf
(but I don't see the fuse box listed...)
I'll let you search the googles for info on the fuse box in your rig. The info is likely out there - you just need to keep searching.