We had a 2005 Airstream with a 2004 W30 chassis.
The ECM on that was on the left side, under the dash I think. It's been a few years, so I may be wrong.
Norman, 2008 Winnebago Adventurer 38J, W24, dozens of small thirsty ponies. Retired after 40 years wrenching on trucks! 2010 Ford Ranger toad with bicycles or KLR 650 in the back. Easy to spot an RVer, they always walk around with a screwdriver or wrench in one hand!