Just some clarification: You get a flat per diem rate for you PCS travel, plus an additional 75% of that for each dependant over a certain age and 50% for dependants under. That is for food and lodging. You then get $.24/mile for gas. Hotel receipts are not required as you get a flat per diem either way, but keep them just in case. So if you go over budget on hotel you pay out of pocket. If under you get to keep it.
Toll receipts are only required if you exceed $75, but again keep them anyway.
Hotel receipts are required for TLE, which must be at the Navy Lodge or you must have a certificate of non availability from their front desk. Note that once you are within 50 miles of Newport you no longer get per diem for lodging but instead must claim TLE and stay at the Navy Lodge. This will be written on your orders as something like "member not authorized per diem in vicinity of destination duty station."
Liberty at OCS is highly dependent on your class team and the policy in the Officer Candidate Regulations that can change, so "when do I get liberty" is impossible to answer. Bank on week 12 and you'll never be disappointed.