Issue could be with the wiring or controls in a wiring/fuse/relay module at the rear of the coach .
Is your coach on a Freightliner or Spartan chassis ?
Either company could provide wiring schematics for your chassis .
Is the neutral relay you're testing in a module like the one pictured below?
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.