Look in entityclasses.xml
Entities have the following 2 properties:
<property name="LootDropEntityClass" value="EntityLootContainerBoss"/> <!--this is the actual loot container that they drop - create you own if you need-->
<property name="LootDropProb" value="0.5"/> <!-- the probability of dropping the above loot bog, in this case 50%-->
I dont know if there is an overall drop probability variable - the above is for each entity.
Edit: You can set a global loot drop probability of individual items but probably not lootbags.
see perkLuckyLooter in progression.xml:
<passive_effect name="LootProb" operation="perc_add" level="1,5" value="1,1" tags="luckyProb">
so you can manipulate the LootProb var as a passive effect in a buff to increase the lootdrop probability of any item using the items tags
As a side note, this post should be in Discussions and Requests.
Cheers