I fly a Diamond DA-40 TDI and you can't check the fuel quantity in the wing tanks in that aircraft either since the filler is not in the middle of the tank but outboard. Diamond delivers the aircraft with a clever little device. It is basically a flat, hard piece of plastic with a cutout in the shape of the front of the wing. You hang that on a specific place on the wing, probably chosen to minimise the effect of unlevel ground. Attached to this piece of plastic is a length of tube which you push into the drain valve. Fuel flows into the tube and up the piece of tube which is riveted to the piece of plastic, until the fuel level in the tanks and in the tube are equal. A scale printed on the piece of plastic then allows you to read off the amount of fuel in the tank.
Should not be too hard to create something similar for a Robin, I'd say.
Just for the record, our club flies Robins too (DR2-120, DR2-160 w/ long range tanks, DR400) and we only check the fuel level through the fuel gauge in the cockpit. {}-(
Oh, a few tips if you're going to fly diesel, and I assume this is the first diesel in your fleet: Get a water/odorproof box and put a club-issued fuel tester and a bunch of gloves (from a fuel station) in it. Leave that box in the aircraft for pilots to use instead of their own stuff. Diesel and Jet-A1 both smell horribly and because it does not evaporate, the smell will last forever. Not something you will want to have in your flightbag. Also, put a empty jerrycan, weighted down with some stones or something, near the parking place of the Robin to discard the sampled fuel in. I don't know what the custom is at your place, but we normally empty our 100LL fuel which we sampled on the concrete to evaporate. If you do that with diesel, it will not evaporate but will get very slippery very quickly.