Do knocked out companions get the full experience

BioWare acknowledged this "problem", but it appears it's just a display issue related to glancing blows:

Glancing Blows are in Dragon Age 2. What this means to the user is that if they do not have overhead damage numbers turned on it will sometimes look like regular attacks do 0 damage (especially to lieutenants and bosses) but this is simply their attack rating being too low and the enemy not being fully affected by the attack. There is a good chance your Hawke doesnt have enough points invested in your main stat (warrior=str, mage=int, rogue=dex) which means your attack rating is too low.

I suspect if you turn on damage popups youll see that you're just hitting for 1-3 damage.

As it's a fire dragon, it's extremely susceptible to cold: stock up on +cold damage items like Cold-Blooded or Mutiny, and try to get the Elemental Weapons spell.

But the fight is supposed to be a long one: it's got a huge amount of health. Health potions probably aren't going to cut it, and you should dedicate one of your party members to be the healer. Also make sure to stock up on fire resistance gear.