This is a timely topic... our lab mix (knocking on the door of 15 years old) also has step issues due to rear hip problems and a bit of general anxiety about the steep, open steps. Not only that, but both last year and this year one of her rear claws got caught/torn in one of the "step holes" and caused substantial bleeding. She only weighs about 35 lbs so we can lift her in and out of the RV easily enough, but she really dislikes being carried.
It does seem like a ramp would be easy enough to make with some 2x4's and textured material attached, but I guess easier portability is what the manufactured items offer.