mod for experience gain in the campaign.

Okay, let's try it.
1) You need to enable the advanced map editor.
In the game folder, find the profiles folder, and in it, the editor.cfg file.
You need the second-to-last line:
setvar map_editor_mode = 1
Change 1 to 0:
setvar map_editor_mode = 0

2) Launch the editor, create a map.

3) We need to create a new auxiliary window.
Attachment 70086

This is what the window will look like; I don't know where it will appear for you.
Attachment 70087Attachment 70088

4) To configure the window, right-click on the two bars; nothing will happen if you click anywhere else.
Attachment 70089

5) Click the "select tables" button. Then, open the window and select AdvMapDesc.
Attachment 70090
Click OK.

6) That's it; the list will contain game maps. Double-click to open them.
Campaign maps are located in the Scenario folder and are marked with numbers, e.g., C2M3 - Campaign 2, Mission 3, Argil invades Irollan.
Attachment 70092

To remove the level cap, go to the map settings and uncheck the box.
Attachment 70091