Question regarding Grey Tank Lever

My Itasca Spirit (29B) isolates the drain valves sufficiently that they can be replaced without doing anything to the waste tanks. My experience generally with Winnebago motorhomes makes me expect that level of care in design of all models. Drain valves go into readily accessible "utility" compartments.

I think you need a better service provider. Freeing up the valve should be first choice, before considering replacement. Replacement of the valve should not normally involve removal and replacement of the waste tank. You've not specified year and model, so I can't look up the plumbing design diagrams.