I have this one (and am very happy with it) »www.ncix.ca/products/?sku=64485&···oid=1448
... it's one of the cheapest gigabit out there and will do what you need.
Manual here »ftp
Plug your RV042 to it, all other LAN connections (direct or from other switches) as well.
Then designate one port as monitoring and connect to it from your sniffer machine running wireshark.
The switch has per-port stats, bandwidth management and more features that you can utilize to manage your issue.
There are more port versions available too (more expensive).
EDIT: There's a deal on this one now »www.ncix.ca/products/?sku=69410&···P%20Link
...mind this is 100meg and only 2 Gb ports. ...I've never used this one so can't provide recommendations.