If you are confident that the leak is due to the valve, I would recommend changing the valve, they are much cheaper than a new toilet.
Fill valve should be on the back of the toilet so as long as you can get in there it should be fairly easy to change without removing toilet.. Otherwise if toilet is too close to wall you will have to remove toilet to change the valve.. I have managed to replace a valve on an older model once that was in a worse place without removing toilet. Wasn't easy but I did get it done.
Kit does include replacement seal, use it if you lift the toilet, do not reuse the old seal as it may not reseal.
See
HERE for parts break down.
Looks like Thetford 31705 Aqua-Magic V Toilet Water Module Assembly would be the valve.
Found water valve
HERE for $35
Found valve replacement instructions
HEREI would recommend adding a braided stainless steel flexible water line to connect the fill valve. Gets rid of any tension that may be present from the stiffer PEX line.. The ones at Home Depot and Lowes will work fine, you need to find one that is 1/2" to 1/2" connections.