I used to have that problem with our prior Pace Arrow. I used to have to crawl under a wheel well, over/around a tire, then reach up, and use my finger as a dip stick. I couldn't see what I was doing when filling, and usually over-filled it, resulting in excess brake fluid coming out of the reservoir when I put the cap on. (The cap had a rubber thing on the underside which took up some of the volume of the reservoir, and pushed out any overfill. I suspect it was there as a way of providing expansion room in the reservoir.)
Hopefully, someone else has a more elegant way of dong it.
BTW nowadays I wouldn't fit in a wheel well along with a tire :-[