Your tanks will be vented, unless you have done something to seal them.
The source of their question is a good one. People will sometimes seal their vents on trailers that are sitting on permanent sites, during the off-season. Maybe to keep odor down, maybe to keep vermin out, whatever the reason may be. Your pump-out contractor just wants to be sure that if you have done anything like that, that you tell them. Yeah it's probably rare, but better that they ask. Their pump is more than capable of flattening your tank like a pancake if the vent is sealed.