Rev dial fluctuating is a sign of a failing TC, but always best to check the atf fluid first ,that it’s the correct fluid and level , u can also use dr tranny fluid as a temp measure
So when you say slipping. Is this something new and it didn’t used to do it? Have you towed with any other Disco 4’s or autos?
An auto box has a fluid coupler called a torque converter. This will flash and allow more slip when loaded such as towing. It should still lock up at cruising speeds. But with some weight behind it will be different to driving normally.
If it is excess slipping the. There could be another issue. But I’d guess more like internal gearbox. It would be odd not to show other symptoms and be fine when not towing though. Does it change gear smoothly or make a whining noise at all?