You've looked and looked and there is no access. The camper's almost certainly designed in one of two ways:
1. They did not plan to service the slide motor.
2. The slide must be removed from the camper to service the slide motor.
Either way, you've either got to cut yourself some access, or remove the slide. I sincerely doubt you want to remove the slide.
Not wanting to cut or tear into the cabinetry, when it is so clear that there is no other way in... You've tied your hands on fixing the problem... That's why I suggested professional help. The pros would pop the slide out and have the problem taken care of in a couple of hours.
