I hope someone with a similar RV will chip in, but an '88 is getting to be a rare one. I'd crawl underneath with flashlight and camera and see what you find. Some have room to get your hand on top and then it's maneuvering the sender and float out through the space provided. If it has a side mounted sender, it should be evident with a look-see.
If the tank is emptied, dropping it shouldn't be too major a job. It will be heavy, so have jacks and props ready, but not an impossible procedure to complete.