Congrats! You've just encountered one of the worst pieces of gratuitously p-poor design ever. Between the poor access to the service door on an underfloor mount and the location of that clamp just below the lower edge of the opening, it's real whirling b---h to get to. No, you're not missing anything easy.
I've got two generations of tool I inherited, and my own collection as well. The only thing that worked was a small pair of right-angle needlenose pliers that Dad (former TV repairman) had. Even with those, I was using the bend in the jaws, not the tip gripping area, to hold the three prongs of the clamp open.
On the Roadtrek, I replaced the Onan spring clamps with screw-drive hose clamps that have a 1/4" hex. Onan should have used these to begin with. When I had the Tiger's Onan out to replace the the fuel pump, I replaced the whole pump-to-filter line with one about 1.5" longer, as well as replacing the clamp. Now I can easily pull it up enough to see the clamp.
A general comment.... I've replaced about three of these filters. I always blow thru the old one in reverse onto a clean paper towel to see what comes out. I've _never_ seen anything but clean gas. I think since the suction line ends 1/4 of the way up in the tank, it's unlikely that it will pick up any trash or water, which usually settle to the bottom of the tank.
Jim, "Mo' coffee!"