Your Multiroom contract explicitly requires all boxes connected to the same phone-line at all times (not just the 1st year). That you don’t is your own business. There are mixed stories on how effectively Sky enforce this (and not charge you the full subscription rate for the subsequent boxes), but 1 thing is common and that’s that you usually get a letter from Sky warning you before they take action.
So only you can decide if it’s worth the risk to bring it on the camper (the risk being slapped with a bill for the subsequent box(es) subsidised portion of the subscription, which might be significant if you have movies or sports). I take my card for weekend trips, and have had phone line issues for week-long periods. I still take my card in the comfort (however ill-informed it may be) of at least getting a warning letter from Sky.
You’ll get much better and broader advice on Digital Spy or Boards.ie