Sleeping in a tent per se is not a problem, so long as you use separate beds and bedclothes or sleeping bags that do not touch. However, Ashkenazi practice is to not to travel alone together on a pleasure trip when the woman is in niddah. In this case, if you cannot delay the trip, we advise being scrupulously careful to keep the other harchakot. For guidance, please see our articles on conduct while in niddah.