score:4
First of all, here is a website that lets you search camp sites across Japan.
Regarding the two places you mentioned, a quick search resulted in this one:
There is a camping site in Iya Valley that lets you stay in a group hut or rent a tent. Here is their pricing list.
There seems to be only one camp ground in Yagen Valley, and it does not offer rentals. I also tried to find rental tents (テントのレンタル) in Mutsu city but could not find any.