If you haven’t already, you need to locate and download a copy of the chassis and coach wiring diagrams for your unit.
Sometimes the coach builder will provide a copy of the coach wiring diagram in the coaches O&M manual.
Our coach uses a 20 amp fuse - powered from one side (un-switched side) of the coach’s battery disconnect relay (fortunately I haven’t had to locate that fuse yet). Power to that relay terminal comes thru a 175 amp fuse located near the coach batteries.
‘91 Ultrastar Champion‘02 Georgie Boy Landau 8.1l Workhorse
‘03 Jeep Wrangler TJ 2018 Jeep Wrangler JK toad