Just went out and checked the size of my chrome jamb nut. I have 19.5 inch tires on a Ford chassis. Size of the jamb nut is 1 1/8th. Use a deep socket as the curved top of the nut holds a shallow socket out off the straight sides.
Just to be certain, take the supplied Winnebago wrench with you to Harbor Freight, use one of their inside calipers to measure the size, then head to the socket area and get what you need.
For the rear tires make sure you have an extension on your ratchet so you can work around the center hub.
I have experienced the jamb nuts being too tight. I am certain some dummy at a shop put them on with an impact wrench.
Safe Travels