What if he were Ranger 1, Paladin 1, Wizard 18... should he have 19th level spell slots?
In that case, I am not too bothered by paladin having slots (his class feature doesn't actually grant paladin spells to fill those slots at level 1). The point (if i am following correctly) is that the paladin and ranger 1 levels "boost" the total slots. So what? It doesn't stretch my imagination that knowing enough about magic as a wizard 18 knows would "boost" paladin and ranger to grant slots, or looking at it another way, that paladin and ranger levels degrade the 20th level characters spellcasting to 19 rather than 18 due to some modicum of spell training (not fully realized, but there).
I don't see it as an issue. What helps to Grok multi-class characters is to think of them as a melting pot, not a mixed salad. The Ranger 1/Paladin 1/Wizard 18 is actually a level 20 "Brokebuild Optimizer" whose class features include spell casting as a level 19 character.