That's a pretty corroded solenoid, Merlin. The device is likely getting relay power but the big contacts inside may be frozen open. If you can get a meter on the large posts of it, you may find that not both posts are hot when power at the switch is applied. It's not an expensive solenoid and should be available at an auto parts store, if you can find one open.
I suppose one could jumper the two large posts, but there may be some sparking, and you'd have to remove the jump just as soon as the pump motor stresses after the slide is in. Regardless, if there is no power at the outlet side of the solenoid when the switch is applied, you'll need a new one.
But as Gerald alluded to, I'd first make sure the problem isn't low fluid in the hydraulic reservoir, a common problem. Make certain the coach has adequate blocking under it if you have to crawl underneath to work.
Joel