Ok, I went down and looked. Above the fuel filler is a plate of plywood and linoleum with phillips screws in it. I had to remove couch away from cover and pull back carpet when I changed tanks. From that access, you can see if there is a blank plate with 5 holes, 2 3/4 in diameter. If you have this, you just need to purchase the sending unit and can most likely insert it down into tank. If you dont have it, you would need to pull tank and have a blank mounting plate soldered, not brazed or welded, to tank. Everything can be disconnected from top of tank through this holeAn older radiator shop should easily accomplish this.The sending mounting plate is just to rear of rectangular fuel filler unit, and centered on top of tank front to back. I have the short tank from this coach and one in the parts coach and neither has a block off plate, so likely, yours is the same.
The tanks are 32 inches tall. The sending unit is stamped on resistor hsg 90 ohms. The round top with 5 off center spacing holes is 2 3/4" diameter. The rigid stem that sticks down into tank is 16" . The float arm is 19" pivot point at resistor to tip of float. Max down 31" , max up 1/2" below underside of float mount. Single threaded post on top.