First, it should definitely be are because that matches with the plural words.
If you saw an ad that used is, it was wrong.
All of these words in me are to express how much I love you.
Although slightly unusual, there is nothing wrong with that sentence.
Variations, including your alternate suggestion and some that don't use are at all, include:
All of these words express how much I love you.
All of these words are to express how much I love you.
All of these words are meant to express how much I love you.
All of these words in me express how much I love you.
All of these words are in me to express how much I love you.
All of these words in me are meant to express how much I love you.
Each of these is grammatical.
I can't tell you which version you should use. It depends on exactly what you're trying to say.