When I run top in memory display mode and when I check the processor usage by day in WHM, mailnull is using about 45% of the memory at all times. mailnull has 4 running processes. Is there any way to cut down on the memory usage to free it up for other things?
mailnull is simply the account that exim processes run under. You need to determine what those processes are. If they're MailScanner processes, then the only way to reduce memory consumption as a whole is to reduce the number of child processes in the MailScanner configuration.
A setting of 3 is usually fine on most servers. You can reduce the number but need to watch for incoming email queuing to be processed. Just because the child processes are using memory isn't necessarily an issue, unless the server has actually started using significant swapfile space.