Oh my god...please try to convince them! They will regret it! You come from so far away...you would miss so much fun! The comfort of a camper/hotel is neither worth all the money...nor will you find out what Wacken really means.
And tell them, they'll feel worn out each day anyway because festivals like that tend to be really exhausting.
Having a real bed doesn't help.
And remind them of the fact that it takes hours to get to the hotel at night and then again it takes hours to get back to Wacken. So you'll either have less sleep than you ought...or miss some bands.
If you can't convince them to use tents, you'd better go for one of the campers. It's still better than a hotel.
Cya