In a node type hotel I have an entity reference field to a node type offer. For the offer node.tpl I need to output some fields of the referencing node type hotel.
I have no idea how to get these fields inside the referenced node.
I'd put this in a comment but have no rep so I can't ...
Create a view of hotels as Robin said. The entity reference module provides views relationships that go in both directions , so add a relationship that corresponds to "entity referenced from field yadda yadda" -- sorry I don't know exactly how it'll be labeled offhand but with some trial and error you should be able to find the right one.
Add a contextual filter using the offer relationship on nid, default to content id from url. Be sure to choose the offer relationship when adding fields.