Best surface for tires to sit on for extended periods?

I park mine on gravel without anything between the tire and gravel with good results.

I use my rig perhaps twice a year, and it sits there otherwise.

My tires are now almost eight years old, and one of them is just now showing signs of tread "rounding" thus indicating incipient belt separation. I'll get a new set before using the RV again, and park them back on the gravel.

It seems to be working OK for me.