G'day,
The Range Rover sounds like a 2006 model so it could be TD6 or V8 diesel or a V8 petrol and the difference may be significant to how you drive it. It could also be a monocoque Range Rover or the Range Rover "Sport" model built on the S3 Discovery platform. The difference is rear overhang and affects handling.
I expect that it has a 5 or 6-speed ZF automatic transmission. Based on 20,000KM per annum it has done 60,000KM. Make sure it is serviced before you do this trip and get a report on condition. The service is just fluid and filter strainer. If the fluid is dirty and strainer is full of muck it might indicate pending disaster. Try to get a drive of the RR before towing so you know the speeds and REVS where the transmission goes into lock-up in each of the higher gears.
Go slow when towing. Go really slow for the first 5KM until the computers calibrate the suspension and brakes or the RR. Then calibrate the trialer braking system according to the makers instructions. It doesn't take the computers this long but you have to get used to the change in how it feels up hill, down hill and around corners and fine tune the trailer brakes. If the systems can't calibrate the weight warning lights will flash and you should park it and go and get the truck.
The 400KM trip at 100Km/Hr takes 4 hours. The same trip at 80Km/Hr takes 5 hours. Traffic conditions and speed limits will narrow this further so trying to go fast will just expose risks for marginal time savings.
Drive in a low gear - not "Drive". Find out which gear is direct top (i.e. 1:1) and use it until you know that the transmission is "locked". If you can maintain that speed shift up one gear.
If you have to slow down pull back a gear and "squeeze" the brakes. ABS brakes on modern vehicles hate big towing loads. If brake calibration is not spot on the trailer will lock up brakes and shove the RR and the ABS on the RR could release the brakes. Maintain lots of distance from the car in front and be prepared to slow down to cope with these idiots.
Try to enjoy a 400KM trip with 6-tonne up by being very patient.
Regards,
White Pointer