Yes, you can safely add a manual ball valve to your '08 Fleetwood Terra's heater line before the electric valve. This will function identically to the electric valve and won't cause flow issues since the system is designed to handle closed valve conditions.
For your E450 V10 with 8-16 miles monthly on dusty dirt roads, change the air filter every 12,000-15,000 miles instead of the manual's 30k recommendation. Check and clean it every 3,000 miles.
Sewer hose without fittings is rare - buy cheap complete kit and swap fittings, or try industrial bulk hose. Consider repairing leak with hose clamp or sewer hose repair tape.