Yes, much nicer to have it reasonably level - for several reasons. Sleeping downhill doesn't suit everyone and eating your meat and three veg off the floor after the pans slip off the stove doesn't add much to the taste either.
Also your three way fridge works better if it is reasonably level. No need to go silly but within 3 degrees of level is OK. (that is about 4" difference side to side (of the van, not the fridge) so that is a fair bit). Less slope the better though.