Yes, you're absolutely on the right track - your testing clearly points to a PCM failure in the ground switching circuit for the drivers side coil bank. The fact that you have +12v but no ground pulse during cranking confirms the PCM isn't completing the circuit.
Starter relay is in engine bay fuse box on Ford 7.3 Godzilla. Starter staying engaged usually means stuck relay contacts. Replace relay - should be warranty covered on 2022 unit.
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.
Your 1988 E350 Class C with 460 EFI has spark, fuel pressure, and timing but won't start even with ether - this points to either a faulty Mass Air Flow sensor, clogged injectors, or ECM failure. The fact that ether doesn't help is the key diagnostic clue here.
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.
Super Tech DEF is identical to Blue DEF - both meet ISO 22241 specifications. Buy whichever is cheaper. All API-certified DEF works the same regardless of brand name.
Avoid deep water with RVs. Water in engine causes catastrophic damage. Know air intake height. If crossed, check all fluids for water contamination.
Contact Gregory Poole CAT dealer for mobile service in eastern NC. CEL after battery failure often requires code clearing with CAT ET software. May be stored codes from voltage event. Engine running smooth means likely safe to drive to service if needed.
This appears to be a faulty parking brake switch or wiring issue causing false signals to the engine control module. The parking brake system shouldn't trigger limp mode under normal operation, indicating an electrical problem that needs diagnosis.
The SPN 1569 code indicates the ECM is monitoring for potential engine protection issues, likely related to your recent SCR system work. While the RV may run normally, I'd recommend having the system recalibrated before your long trip home.