The ignition relay on a 1988 Ford E350 (Fleetwood Tioga base) is typically located in the engine bay fuse/relay box on the driver's side firewall, or in a separate relay box near the battery. Some models may have it mounted on the firewall or inner fender well.
Essential guide for Ford E-Series Class C motorhome chassis covering suspension, maintenance, and common issues.
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.