Another very important reason to "level" your rv is for the fridge. Use it enough times out of level and you will be replacing it. You should have the following to start with in your inventory:
1. leveling blocks or 2x6x12-15" ones(4)with the end of each cut at a 45 degree angle to drive up on. Maybe (2) 4x4x12" for placing under the tongue. Wheel chocks as others have mentioned.
2. 6-8 inch level from Lowes, Home Depot, hardware store to assist in leveling your rv. Don't get the cheapo round $2-3 ones.
3. Digital thermometer with a remote for the inside of the fridge to monitor temps.
4. 30a or 50a surge protector depending what you rig setup is.
5. Good tire pressure gauge.