Need more info…
Pex fittings are had at homedepot. There are two types, 1 press-in fittings, and 2. crimp. The press are cheaper and likely what a camper builder would use. These are easy to replace and you find the tool at homedepot. no need to describe it here since u have to go buy one anyway. it is a very simple tool, cheap too. The fitting might be fine, maybe it just needs to be pushed in more. You can push the PEX tubing into the fitting with no tools, Just shove it in more.
If it leaks
then u may need to replace it.
You might need to remove toilet, but that is not hard at all. Two bolts just like you house toilet.
If it is a crimp fitting then it more work, but can be done yourself. Tools much more expensive, see if you can rent them. You will know the crimp method as you see a brass ring around the PEX tubing at the brass fitting. You will need to cut the PEX, then cut the brass ring (special too) and then a special tool to crimp a new ring on. Unlikely that a crimp would leak, so I am suspecting the slide-in fitting, or maybe the old-school plastic screw together ones.