Window condensation is a common problem, but as has been said, it is easy to get rid of, and may even go on its own accord once the van get sunlight on the windows for a few hours.
Caravan windows, whilst double glazed, are not sealed units such as you get in domestic use.
The smell in the toilet is worrying.
A clean toilet should not smell, and if it is the toilet, why is the seller allowing it to be sold in that condition?
Have a really good close look at the walls, window surrounds, corners etc for any sign of wetness, or bubbling wallboard, or missing sealant around fixings and fittings.
I would try and find a local mobile engineer to give the van a damp check for you.
Ask for servicing details, check tyre age and condition and get the seller to show you every single item in working order, get them to show you the fridge lights on gas and works on electric, get them to light the fire and test the blow air, and also the water heater, pump, taps etc.
If they cannot show you a recent service proof, be very careful an ddo not commit until you are fully satisfied.
See what reaction you get to suggesting a mobile engineer does a pre purchase inspection..if they say no, say good bye