The aftermarket trailer brake controller's red wire spliced into your steering column is likely interfering with the cruise control system. You can safely remove this connection, but the brake controller will need a proper 12V power source to continue functioning.
Class A offers more space and amenities at 28-33ft. Class C easier to drive with better MPG. For your needs (king bed, large shower, recliners, tow SUV), Class A likely better fit. Test drive both before deciding.
Your 2025 Thor RV on a Ford chassis requires a brake controller to operate trailer brakes safely. Most Ford chassis come pre-wired, so you'll need to install a compatible controller like the Tekonsha or CURT models and connect it to the existing wiring harness.
The 2008 Monaco Diplomat is generally a solid Class A choice, but focus your inspection on the Caterpillar engine maintenance records, slide-out operation, and electrical systems. Verify all major maintenance has proper documentation and consider a professional pre-purchase inspection.