Your P0581 code diagnosis is correct - the ECU is detecting high voltage on the cruise control switch circuit. Since you've eliminated external wiring and switches, the PCM likely has an internal fault requiring professional repair or replacement.
Air bags dropping with tanks holding after recharge indicates leak between tanks and bags - check height control valves and air lines. Spray soapy water on fittings to find bubbles.
Connect bus via existing 30A inlet with transfer switch in generator position. Transfer switch must fully isolate from grid. 3000W Victron can provide about 25A continuous. Monitor battery carefully - house loads can drain bank quickly.
LED clearance lights turning off when headlights activate is a ground issue. LEDs draw little current so poor grounds cause backfeed. Fix by improving ground connections.