On my 2000 Eagle (the same year as your Designer), I have two sets of two switches- One set is in the bathroom above the sink and the other set is in the galley under the control center (the monitor for your fresh/gray/black tanks and battery).
One switch operates the water pump and the other is for operating the hot water heater (HWH) on gas. Both are 12V switches and have red indicator lights that illuminate in both the bathroom and galley when turned on from either location.
The switch for running the HWH on electric is on the HWH. If you open the inside access door for the HWH, you should see a small rectangular box attached to the side of the HWH and has an on/off rocker switch on the top. I can't say for sure, but being that the Designer series was Jayco's top-of-the-line trailer that year, they might have put the HWH's electric on/off switch in a more convenient location somewhere else in the trailer. For the real answer, contact Jayco Customer Support. They'll know.
According to my 2000 Eagle brochure, there were two HWH types offered on Eagles. Mine is the optional auto-ignition gas/electric. The standard was gas only with auto-ignition. Although the Designer series was a more upscale trailer than the Eagle that year, I have no documentation to determine what HWHs were standard/optional. Maybe someone else knows.
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TV: 2009 GMC Sierra 2500HD | Crew Cab | Std. Box | 4WD | Duramax/Allison
RV: 2000 Jayco Eagle 266 | FBS | TT -- 1986 Coleman Laramie pop-up -- Still in the family!!!