One of the big issues is dry rot. Tires will dry rot in roughly 4 years when not used. They actually last much long when used periodically because heat brings protective chemicals to the surface of the tires.
Sunlight merely hastens the deterioration, but air alone will cause oxidation and dry rot. Typical tire dressings are cosmetic and may actually harm the tires. Aerospace 303 is water based, does not harm tires and may help prolong tire life. Before application you would need to wash the tires thoroughly on all sides. Spray on 303 and rub in on to every bit of the tire surface (excluding the tread). You need to re-apply every few months especially on the outside surfaces where rain water will wash of the treatment.
In any case carefully check your tires before the first trip and keep an eye on them for cracking after you start to use them.
Have you pulled the batteries and periodically check the charge?
Do you have a generator stored separately and used at least a few times a year?
I would also inspect every square inch inside and out checking for leaks or sealant issues.