It's not that hard really, I recommend it. RV rentals are a thing in Japan. I am from USA, so I went to a local shop here that offers International Driver's Permits. All they do is look at your driver's license to ensure it's valid, and then stamp a little paper brochure that has a bunch of translations of what your country permits with a driver's license. You can get as many of these as you want, it's not an actual license but just a certified translation, so you still need to carry your regular license. I got pulled over once at 3am in the middle of nowhere and the officer didn't speak any English and was so confused when I handed him my documents but once I pointed to the Japanese section of the IDP he studied it for a bit and then made the sounds of "ohhhh, I get it now" and let me go.

(Aside: If you're traveling to a country where casual roadside police bribes are a fact of life, (NOT Japan, more like Turkmenistan) multiple IDPs can be really handy, since the mechanism of the bribe is often 'I will hold your important document for you until you are ready to give the money' and they may not realize the IDP isn't actually your license, so you can just drive away.)

Also, language barrier isn't really an issue day to day. Most people in the huge urban centers (such as where you'd pick up the RV) speak English well. Once you get off the beaten track, English is a lot less common but translation apps work well (i enabled the Japanese keyboard on my iPhone and often we'd just pass the phone back and forth, typing messages) locals off the beaten path tend to be interested in foreign travelers, happy to take time to help, learn more about you, etc. Small regions in Japan tend to have strong local identity and there is generally an interest in sharing that with travelers.

I only did it for 2 weeks but Japan would have let me stay for 90 days without a visa.