I wouldn't bother putting the stabil in first. I think that would be just wasting it. Make sure and lift the front of the trailer up as high as you can, normally the pickup is in the rear of the tank. Then flush like you said and that should get a lot of it out. Myself I would drain it then put a couple of gallons in and drive it around, then pump that out. Make sure and pump fresh through the line using the bulb to flush it and the pickup also. Stabil I don't think will dissolve varnish I think it just keeps it from forming. I hate to say it but if you want to melt the varnish some type of alcohol product usually works best.
