How to assign MongoDB the complete RAM of system?
You don't need to do anything. MongoD -process will use all available memory automatically. You may not "notice" it, because mongod -process don't eat all your memory, but in reality, all free memory is used to cache data files (data itself and indexes; weight is on indexes). That's why, available memory should be bigger than active data and indexes are.

If you use wiredTiger, data and indexes stays compressed in the memory too and not only on the data-store.