What is the best process for recovering vehicle diminished value from accident when other party's insurance property damage limit has been met?

Wife was involved in accident 4 JAN 13, not at fault, other party 100% at fault. Other party's insurance policy had $25,000 property damage limit, and had offered $4278, bal of property damage limit available after repairs and rental car fees. We believe this is much too low for our 2012 Volvo S80 which was less than 8 mos old and only had 5386 miles. The other party's insurance company, Agency Ins., had an appraisal performed and their estimated DV loss was $7100, but had listed our vehicle as "average" when it was definitely in "excellent" condition and DV would have been much greater. After attempting to recover this DV from both Agency Ins. and our insurance policy (USAA) uninsured motorist/underinsured motorist clause, USAA indicated the $4278 offered by Agency was sufficient and had no more to offer and backed away. Then a bill of $2687 was presented to Agency for the damages to Street Light Pole and Commuter sign. Agency paid this and is now offering just $1591 to settle.