As far as I saw, they required cars to come back 70% charged, or to the same level the car had when it went out.

Yes. This was the policy I saw when I considered renting an EV in Phoenix. We were only going to be there for 27 hours and our hotel did not have its Level 2 charger operational yet, so we opted for a gas car instead.

I rented a Bolt from Hertz last December and their policy at the time was:

“Return your EV at the same charge level as pick-up and pay $0. Or return your EV at any level for a $25 Recharge Fee.”

They were pretty lenient about the return charge — it was close to 100% to start and 80% when I returned it, but they didn’t charge me the $25. Even if they had, the EV rental rate was cheap enough that I would’ve still come out ahead vs. renting a gas car.

Meanwhile, I'm trying to find the answer to this on a hypothetical hertz.fr RV rental.

They have a gas policy section, which says "Si vous avez loué un véhicule électrique, veuillez vous reporter à la section Véhicules électriques".

In other words, find and check the electric vehicle section. But they don't have a Véhicules électriques section...