Two problems: removing the old decals and getting something to replace them.
As Tom says, the removal problem has been discussed here often and its a bear at best. Basic choices are to apply heat (heat gun) and carefully lift the decal with a wood or plastic scraper, or to use a decal removal solvent from 3M or Klean-strip. Or combine both techniques.
Replacing them means either a custom painted trim or having am automotive sign shop make a custom decal (called a "wrap") for you. Google 'RV decal replacement' for some sources. I would suggest having the desired pattern painted on, to avoid having to replace decals again in the future.