In a case like this, more useful to post the Make/Model of Toilet vs. of the RV that it's installed in. Which toilet do you have?
Are you sure the little plastic fill valve in the toilet itself was winterized? Either by flushing antifreeze through it, or blowing it with air till dry?
Sounds to me like that valve froze and is damaged, but by all means check lubrication first.
If it's damaged, the valve could drip into the floor and soften/rot it.
If God's Your Co-Pilot Move Over, jd
2003 Jayco Escapade 31A on 2002 Ford E450 V10 4R100 218" WB