Thetford Toilet

Hi, I ordered an new thetford toilet. I was told I needed a low profile toilet, yet when it arrived it was High profile thetford. I called the company and they insist that this toilet will fit correctly in my 1978 Airstream Excella motorhome. is anyone knowledge regarding this matter. I placed the toilet over the hole for the old one and it is so tall. I would have to help my granddaughter up onto the toilet. This doesn't seem correct to me, can anyone please advise me.

Thank you and Best Regards

Norge Moose (Leticia's Popsey