The old question of what size truck do I need

I'm asking this question for a friend as I'm a truck camper and know squat about towing a fifth wheel. I need your expert advice.

The friend has a toy hauler which can be converted into a horse trailer which can weigh as much as 19k lbs.

He's looking for a used truck but doesn't know what make, diesel or gas, engine size, rear end ratio, etc. he would need to pull that weight.

Thanks for any help you can offer.