When your RV's washer P-trap dries out from non-use, gray tank odors will escape through the drain. Add water monthly, install a trap primer, or use a P-trap cap to maintain the water seal and prevent sewer gases.
Your Cummins 400's sudden power loss and throttle unresponsiveness, especially in hot conditions like Death Valley, could indicate ECM failure, fuel system problems, or turbocharger issues. Start with diagnostic scanning and check fuel filters.
Avoid deep water with RVs. Water in engine causes catastrophic damage. Know air intake height. If crossed, check all fluids for water contamination.
Odor while traveling is pressure differential pushing tank air back into coach. Check vent for blockage. Add powered vent fan or 360 Siphon cap to solve. Also check Vacuflush ball seal for wear allowing odor bypass.