Thor Ace has both macerator and gravity dump. Look for valves in the wet bay that direct flow to one or the other. Close the valve to the outlet you are not using. Some setups have both connected - gravity works by default, macerator works when pump is on.
Generac oil drips after breather hose replacement suggest other issues: check oil level, breather valve, hose routing, and valve seals. Clean engine and observe where oil first appears.
For your 2000 E450 V10's uphill struggles in the Rockies while towing, rear end gear change will provide the most significant improvement. An SCT programmer offers moderate gains, while K&N filters and cat removal provide minimal power increases.
Your RV sewer hose will likely freeze at 25°F, especially with the uphill section that creates standing water. The two-inch rise at your hookup point is particularly vulnerable to freezing.
Yes, there are aftermarket transmission controllers available for your 2007 E450 that can hold it in Drive gear. The most practical solutions include installing a manual shift valve body kit or an electronic transmission controller.