What is your tow vehicle?
There will be a pressure and a return line to the cooler. You should be able to remove the pressure line at the transmission and add a "T" fitting where you can install the sending unit along with the pressure line for cooling. The pressure line should probably be the hottest temperature point.
I have a 2002 GMC Safari pulling a 93 16' Casita
Spirit. I haven't installed the transmission temperature gauge yet. I do believe there may be an OBD2 code that feeds the transmission temperature as I've found the flashing "Service Engine Soon" (SES) light
when going up hill in hot weather.
I would question cutting the rubber hose to add the sending unit. Oil leaks
as well as heat are not good unless you have a deep pocket book.