There is damp under the vinyl flooring, and it's trapped there so cannot escape/evaporate/dry out.
So realistically there are the following options open to the OP.
1. Ignore the problem and hope it goes away (we all know that it wont)
2. Leave it until the floor is rotten and costs a fortune to repair. (See above)
3. Trade it in for something else and pay thousands for the privilege.
4. Accept that the vinyl needs to come up to investigate how much damp there actually is, get it dried out and fit some new flooring, either yourself or pay a carpet fitter. (Much cheaper than options 2 or 3)
The OP has asked for advice regarding the issue. That advice has been freely offered by many, along with one personal experience of its possible (probable?) cause and remedy.
It is now up to the OP what, if any, advice they take on board and how they now proceed. Horses, water, leading??
My caravan originally had the "Rhino hide" flooring, which we hated. After my trauma I fitted different vinyl flooring throughout the caravan myself ( not a quick job!) which, we think, looks far nicer than the original (its much thicker quality and softer underfoot) Cost was about £90 including a few rolls of carpet tape.
