Different text for mobile page

Hi,

I have a problem with this topic. I need page with two different contents. One for desktop browser using users and second for the people accessing my site via mobile similiar device.
The simpliest aproach for this seems to me is to create a new field, like the main content field of post and add there different content (totaly different, so just hidding elements is not enought). Is this possible?
Where should I add this field (I know in DB, but also in which page should I add code to make sure,that is correctly saved and loaded in edit mode)? I created my own templates, so I could show the page it in my way. Just to make it work in admin section and in the core.
I found one topic about this, but it seems to me as unresolved.

I would like to avoid installing this new things.

Thanks