Water drops on inside of roof

Thanks in advance to anyone who can help. We are new(ish) to caravanning and have purchased a 2003 Avondale Dart 556/6 earlier this year. It has been a great van and remained dry through many a summer downpour. 

We keep the van in storage and decided to check it earlier in the week after quite a few days of hard frost (lowest temp being -5 and highest being 0 over the course of a few days). The temperature picked up the night before I went to see the van and when I checked it all the frost had dispersed.

I found a considerable number of water drops on the inside of the roof (no puddles, soggy areas or signs of leaks). There were sweat lines every 50cm or so on the roof over the back 2/3 of the van which seemed to coincide with where the curtain rail is screwed into the roof.

Has anyone came across this before and if so what was the cause and how did they manage to remedy it (if at all). I'm hoping that it was just condensation from the sudden frost though I thought it prudent to ask the advice of more experience caravanners than ourselves.

Hopefully this link https://www.dropbox.com/sh/3ztt8b9s7mus910/AAAiCuBbJ8fs8SJE07GfyNRRa?dl=0 works as it is photos of what I am trying to describe. After towel drying the roof I did lower the front and a little water did drain off from the roof.

Thanks for reading (and hopefully helping).