That's a good idea. You dont even need diag_memory_usage, you can just observe the sys_memory in the F11 panel and see if your level leaves some memory unfreed.

Also check out the acklog, it normally tells pretty good where the problem happens. If its an entity in your level, you can put a diag output at the start of the entity action, then you see at least which entities load fine and do not use up all that memory.