When your drone is in RTH mode and it goes 20 meters, will it keep Parallel to the ground up and down large hills or will it fly into the side of a hill?

Asking the same question 3 times won't change the answer.
I'll see if I can make the answer clear this time.

RTH is going to be level at the height you set relative to the home point.
The only way it will be parallel with the ground is if you are flying over a pancake flat plain.
It's not going to go up and down with the terrain because the Phantom has no way to know what the ground is doing under it.
There is no sensor measuring height above the ground.

It will hit the side of a hill if its lower than the top, that should be clear as glass

Depence a little bit, if you have what I have, PH3P, then it won’t care. You have manually try to avoid any thing in the flight, RTH I have set at 45meters, because I want to avoid any trees, then I can keep a tap on the landing only.

However, PH4 and above, that’s another story and it’s getting better every time there is a new equipment out.