I doubt it will have any impact at all. For Stafford loans, they don't even look at your credit report, and GradPlus just does a cursory review for negative information. Private loans might be a different story, though.
That being said, it's a really, really bad idea to cosign for any loan you aren't willing and able to pay yourself. But you knew that already. Come on--an RV? It's not like you're cosigning on a basic car to get he back and forth to work. This is something that she needs to figure out on her own, or do without. That's the opinion of this anonymous internet poster, anyway.