Not sure if this will help anyone but here goes -
If you want to include approval emails in the activity history you first need to create a new relationship in FRS_ApprovalVoteTracking to Journal. This appears to be because the existing rollup is set to look at the journal business object so that it pulls through both emails and notes. The current relationship in FRS_ApprovalVoteTracking looks at the journal.email business object and is unsuitable. Set it up as follows -
You can then add to the existing activity history rollup by navigating to the layout > form and selecting the object link (journal) next to Activity History in the Child Panels section.
Add a new rollup sequence navigating from ServiceReq to Approval to approval vote tracking and finally to the new relationship.
Save and when you look at the activity history now for a request which requires appravl you will see the emails that were sent for the approvals.