Gravel, Asphalt or Concrete parking spot??

I will go with gravel as better than asphalt. Asphalt (For sinking in reasons) is my last choice, and since one place I park has asphalt sites I've seen the evidence of that.

Gravel drains, this too can be an advantage.. Concrete may drain if graded properly. When working under the RV I prefer Concrete, but .. Well, I do not always work under it.

Don't forget to add a second cleanout "to make it easier for plumbers to diagnose problems" since most areas do not permit rv sewer hookups.

X2. Some treated boards under the tires will reduce moisture and dry rot, regardless the hard surface. Good luck.

Concrete is the best option then asphalt is next. The tires and jacks can sink into asphalt on really hot days but a couple of boards to spread out the foot print helps that. If you ever need to go under the trailer for maintenance or repair you will be glad to have any solid surface. You will also want to make it wider than the trailer for maneuvering angles and protection from the mower. It is also nice to step out onto concrete while loading and unloading. While making your parking pad think about having water and power installed, they are so nice to have right there.