Let's say we borrow an RV...

What kind of drivers license? International? Contact the insurance company and work out the details.

A separate document can't hurt but it will have absolutely no recognition or legal standing. But with all of the owneres contact information it could be useful along with the registration.

Not sure why people make things more convoluted than it has to be. Borrow it and if anyone of authority questions ownership, call your friend and let the cop ask if it's stolen. Make it easy for yourself, you don't need a notorized paper or permission slip.

if you are stopped for a traffic violation the officer will want three things right off the bat...driver's license, vehicle registration and proof of insurance. when the name and address on your driver's license (you) does not match the name and address on the registration (your friend) the officer will have questions. in most cases a verbal explanation may be all that is required when the officer asks "do you have permission to drive this vehicle" or "does the owner know you have this vehicle". but it wouldn't hurt to have a signed and notarized written statement from the owner with you explaining the circumstances and including his contact phone number.

enjoy your trip!

I'd seriously look into the insurance first. Your friend should also to protect his interests.

Do you have a USA drivers license? You friends insurance may not cover anyone driving his vehicle without a 'valid' USA drivers license. And your insurance from another country may not be honored or acceptable in the USA.

You should just need a note from the owner that is on the vehicle registration. In the event of any legal issues, both of you are covered. Insurance is a must and needs to show the VIN number and your name.
Have a good trip!

remember if something breaks YOU FIX IT!!!! no matter what it is, bring it back better than when you borrowed it. have a great time.

"Don't need no steenkin permission letter" Seriously you shouldn't need any thing but your friends blessing. Double check insurance requirements, but mine covers "occasional operators" just the same.
If you are stopped for some reason unless there is a stolen vehicle report no problem.
If you are a belt and suspenders kind of person, I suppose that a permission slip with a copy of the registration wouldn't hurt,but I am betting that it will never come out of the glove box.

They are both right. Its not illegal to drive someone else's car, but there could be circumstances where it is in your favor to produce a piece of paper that explains it is a legal and voluntary rental or loan. Either way, have fun. It sounds like it could be a great trip!

A very simple permission letter bearing the same signature as a photocopy of the rv's ownership/registration you would carry would save a bunch of explaining.

Totally!
As long as you're able to legally drive in the US you don't even need a contract with the friend as far as the cops are concerned. Only consideration would be insurance and if it's covered with someone other than the registered owners driving it.
Otherwise hop in and go. It's not illegal to drive someone else's vehicle without a permission slip, unless you stole it! Haha