"You must eat more."
Translation:Vous devez manger davantage.
No difference in this context. You may want to use "davantage" instead of "plus" if there is any risk of confusion with "ne... plus" (although pronunciation of the later is plu, while the former is pluss), particularly in speech, because we very often skip "ne".
ex: "j'en ai plus" : only the final SS sound should mean that your mean "more" and not "any more" (pronounced PLU)
Dois is the conjugated verb (linguistically known as a modal verb), so a verb after it must be in the infinitive, very similar to pouvoir and vouloir. Per ex) Je dois y aller maintenant. (I have to go there now.); Voulez-vous parler français? (Do you want to speak French?); etc.