Very coincidental
What an interesting coincidence in that my wife's RV-3 just had this exact problem. We would get to the airport and a puddle of brake fluid would be under the center of the plane dripping from the fuel vent. To get behind the firewall required removing the upper skin and windshield (hers is a slider), then remove the fuel tank. Once that's out everything is easy to see and work on. Hers had a loose tee fitting on the back of the reservoir so we just resealed it and tightened it up then bled the system.
To remove the tank, once the skin and windshield are off, you'll need to drain the tank completely, remove the vent line from the front of the tank between it and the firewall, then remove the fuel feed line at the bottom rear of the tank (unless you have an inverted fuel system with a flop tube, which we installed on my wife's plane. In this case the fuel line is located at the front bottom of the tank and will probably require you to lift the tank up and have a helper loosen the line.) Tank should just lift right out. To reinstall, loosen the tank straps slightly to make it easier to get the screws started on the upper skin. You'll have to sit in the seat when it's all back together, reach under the tank, and tighten them back up. They pull the tank up against the skin.
Do any other needed maintenance under there while the tank is out. It's not that hard, but I don't like pulling it unless really necessary. There's great access to everything up front with it out so take the opportunity to look over everything really well. It's a great time to rebuild or replace the brake master cylinders, tidy up wiring, or install any cool avionics. Very easy to get to everything.