Fixing damp rot on my Sprite?

I have an '88 sprite musketeer XL, i believe it has damp rot in the back 'bathroom' offside corner, and was just wondering if drying the van out like some people have suggested with a heater would solve the problem, or whether it needs treating professionally for safety reasons. I have used the van for many years on the continent, but havent used it in 3 years and wanted to make sure it was safe for the occassional use on weekends in the lakes etc.

Thanks,