Exterior lights on house battery!?

1996 Gulfstream Class C on E350 chassis. After 5 hours of driving at night without the generator running (not something I have previously done in the 2 years/10K miles I have owned it), I noticed in my side mirrors that the running lights on both sides at the rear of the coach were dimming considerably. Pulled over and discovered that ALL exterior lights EXCEPT for headlights, brake lights and the front running lights/turn signals were very dim. Fired up the generator and all instantly brightened back up. So... two separate 12V circuits route not only through the standard, exterior light pull out switch in the dash, but also to the brake light bulbs (engaged brake filament in bulb is still bright without generator running). My mastery of electrical systems ends at replacing fuses and splicing wires, but not having many of the exterior lights (including rear turn signals and running lights) on the circuit fed by the alternator? That's messed up. Chassis mechanic I trust says the time it would take him to be sure he knew what he was doing switching things around (so no problems with generator/alternator current going where it shouldn't) could easily add up to a substantial amount, so neither of us is anxious for him to get started. Obviously I can drive at night if I run the generator, but it's silly to be dependent on that. Not to mention that if the generator is out and the lights drain the house battery, I lose the fridge (running on propane uses electronic flame regulator). Any advice from the veteran RVers out there? And I know I can't be the first to wonder this, but since alternators and generators can both quit working at the most inconvenient times, if there is one of each on board, why aren't RVs routinely wired so that all 12V systems can be powered as long as one of the two is working? The cost of some additional wire and switches on a unit that sells new for $100K or much more is nothing, and the advantages are huge.