BTDT. This 1991 MH had a 1996 solar set on it from back then. It was a farce. The "50w" panel was only 32 cells instead of 36. The controller was a joke.
To upgrade, the easiest way is to use the existing panel as a platform and strap the new one (could go to 160w and be ok) on top and give it a new controller.
The new controller should not be attached to the back of the fridge on the way to the batteries using the existing controller's wiring. (as is likely with yours) Do it right, and with proper gauge wire.