A toilet leaking at the valve could be gasket failure or improper winterizing, allowing ice to crack the valve body. In either case, see if a replacement kit or valve is available. Often the best first step is to remove the toilet, 'sterilize' and replace parts on the workbench. It's made by Thetford, so I believe parts are available for most models. By the way, some Thetford toilets have a cover over the bottom part, hiding the bolts. Remove the cover or shroud first to aid in removal.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI