In fact these days the 'general' rule for names in neolatin is that the last name is not latinized anymore. The first name however is latinized. So to avoid every confusion in a declension you sould always add the first name.
If however - like it was done before- also the last name is latinized, there is no rule for a declension to use: every name is a case on itself.
Mostly it was like that that in neolatin texts the first one to use a historical name, chooses the declension.
For Hitler, mostly the name is not latinized, but there is actually a latinized last name. I saw that even the Latin Vicipaedia is sometimes in doubt and they ask the same question you do. In other threats they seem to be decided and choose Hitler, -Hitleri, 2nd declension. I see thet Hitler, Hitleri is most frequent, if latinized. Although there seem to be a preference for the non-latinized form.
An example of the non-latinized last name can be found in a recent nuntii latini news message:
Pellicula de Adolfo Hitler
In Germania pellicula cinematographica producta est, quae dictatorem Adolfum Hitler in munimento subterraneo Berolinensi ultimis vitae diebus degentem facit. Quod opus nomine ?Der Untergang? a criticis quidem laudatur, sed multi Germani naturam dictatoris in illa taeniola nimis humanam figuratam esse reprehendunt. Bernd Eichinger autem, productor pelliculae, respondit sibi ne propositum quidem fuisse spectaculo suo Adolfum Hitler diabolum describere.
So I wouldn't latinize it and if you do anyway: Hitler, Hitleri.