Here's my sequence of events for this quest

Went to Kahn base camp

Got quest from Regis after my conversation with Papa and leaving the house.

Exposed Karl's real opinion about the Kahns in frot of Papa Kahn and he was killed by his men in the house. Convinced Mellisa and completed her little side quest about retrieving some chems and convinced her stop the alliance with the Legion

Went to Kahn base camp, convinced Jack and Diane to speak out against the proposed Legion Alliance and got them start making more helpful chems.

I then follow the quest marker to the house and it's directing me to the evidence located in the footlocker but when I retrieve it all the quest markers disappear and I get no new dialog options when talking to Regis or Papa and the game still says I need to find evidence. Just freakin great!

Same here, same order also.

I am pretty sure this has something to do with a certain book that is on a table at caesers tent in the fort. I could be wrong as i completed this quest many game hours and a couple of weeks ago. i KNOW i picked up a book in caesers tent to do with something.

Actually i just remembered that its a slave ledger book that you need to convince the other important Great Kahn ' Karl '. It looks like a pre-war book and if i remember its something you can take without stealing.

As Blue 4 Eva said, however the wiki also states that if you take the journal from Karl's footlocker and show it to Papa Khan with Regis present, that works the same as confronting Karl (without the need for high Speech skill) but has the added bonus of convincing Regis without needing to get the book from Caesar's tent.