Jim, in my 05 Diplomat and most other Monacos that I know about, the low beams (single filament bulbs) came on when the headlights were on (outer bulb position) and they went out when the high beans were on and the inner bulb position illuminated, again single filament. Many of us rewired the lights so that all four came on when the high beams were used.
You may want to check and see if someone reworked the low beams and added a different socket (dual filament), using the high beam wiring to fire the dual filament. It doesn't make electrical sense to do it that way, but who knows.
Soooo....either they were wired different in 2003 or someone did some rewiring.
Don & Mary
2019 Newmar Dutch Star 4018 - All Electric
2019 Ford Raptor Crew Cab