I know you mentioned about 7 inches of the floor is rotted. I assume but, you post doesn't say you already removed the floor covering? If so, are you sure it's contained to only 7 inches. If it is only 7 inches, as long as you have good support underneath which you should, your 1x12 idea should work. If it's a larger area, you might want to move towards 1/2 or 3/4 plywood or something like that and do the same idea, pan it to meet the floor height. Really anything should work that's going to give the floor a solid feel once you got all the rot taken care of.
Everyone on here is happy to answer questions for the most part so I'm sure you'll get alot of other responses too, perhaps people who had similar experiences.
By the way, welcome to the forum!
2012 Keystone Sprinter
Still sticking around for the good people and advice.