Don't use a tarp to cover your TT. Use a cover specifically made for that purpose such as ADCO - found at Camping World or http://www.adcoprod.com/.
I wouldn't park on dirt - why? Because when it rains dirt splashes up and covers everything. Use the 12x12 pavers as suggested or make a big enough wood form, excavate the area where the tires will sit insert the form at ground level and pour in some kwickqrete from Lowes, etc.
As stated, use tire covers.
Get some Damp Rid containers - also at lowes, Walmart and put them in the TT. Some will say to crack windows or vents, but to me this just lets in damp air which is what you don't want. Walmart sells the big buckets of damp rid near the camping stuff which isn't what you want at first. You need smaller containers of it - go to the food section and get the ones from the cleaning products aisle. I saw better ones at Lowes for the same price.
