I'll have to assume your looking for the engine ECM
Both Cummins and Caterpillar mount their ECM on the intake manifold side of the engine . In a diesel pusher , that's the passenger side.
If you require more info ; please add your engine size ( ISB ? C-7 ?) and chassis manufacturer to your post .
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.