**park somewhere level, now install a pair of bubble levels inside your rv where you can see them from your drivers seat. Make sure you are level front to rear and side to side.
Having these inexpensive bubble levels installed will allow you to position your rv on sites to the point of being very close to level if not level. Doing this has reduced my need for any additional leveling.
If you need to stop slipping.
Get yourself some rubber sheeting. Cut floor mats to the size you need and place them under the wood or plastic.
Make an incline like a driveway apron. IMO, a 4x4 length, the width of your rear tires and a double wide set of ramps will work.
If that doesn't lift your rv to a comfortable point then you need to find another site or grab a shovel and dig holes for the tires to drop into to lower the needed side.