I am not a small person, but I have never had any real problem reaching in to attach the hose or tighten the clamp.
Once you determine for sure that that is the source of the leak, and you still find the you can't get your arm in to access the hose and clamps.... Try this...
Drill out the rivets at the external filler. Then you can pull it out far enough to attach and tighten the clamp.(the hose will extend far enough)
I recently changed the type of filler device on my personal Scamp
and I really like the new design. There are several on the market which work fine, but I liked the one shown below, not only does it work well, but I had a couple of them in stock!(lol)