The mud flap / wheel liner is a known problem on the Wayfarer. Many weren't secured properly at the factory. It's a shame you lost one before being able to secure it properly. You can order a new one from Tiffin. Note that it is much easier to install the new one with the outer wheel removed.
Your slide problem may have to do with inadequate power. When extending or retracting the slide it is best to have a source of power other than just the batteries. So, either have the generator going, be plugged into shore power, or have the engine running.
Randy and Tina & pups: Cinnamon & Cookie (RIP Coco
2020 Tiffin Wayfarer RW
Mercedes Benz 2019 Sprinter Chassis