The dash lighting dimmer switch right next to the headlight switch turns the reading lamp on when you turn it past full bright.
I just checked the brochure that cam with my 2004 Sightseer 35N and I don't see an option for an electric water heater.
You can download a copy of the manual here: Previous Model Year Product Information
Wiring and plumbing diagrams are here:Manuals & Diagrams
As a side note I added another reading lamp like the one above the driver to the passenger side and mounted a wall switch down low so the passenger can easily reach it.
I bought LED bulbs to replace the halogen bulbs in those two reading lights.
LED bulbs don't work well in the other reading lights in the coach because the beam is smaller than the halogen bulb and the other fixtures don't swivel enough to put the beam where you need it.
Also you might want to check and see when your chassis was built. It is normal for 2004 coaches to be built on a year earlier chassis.
Although my coach is a 2004 model year the chassis was built in May of 2003.
In August of 2003, Workhorse changed the front stabilizer bar from a 2 inch to a 2.5 inch. That made a major improvement in roll.
I ordered the 2.5 inch bar and installed it and I would highly recommend you do too if you have the 2 inch bar.