As you will have noticed, the dative plural of the definite article is "den", regardless of the gender of the noun.
All other determiners (i.e. articles, possessives and all other items that "determine" adjective endings) also end in "-n" in the dative plural - i.e. diesen, meinen, deinen, seinen, ihren, unseren, euren, Ihren, allen, sämtlichen, welchen etc.
All adjective endings in the dative plural also end in "-en" - regardless of whether they are in the weak, mixed or strong declension:
The German flag Grammar 2: Adjective endings in the dative plural
Weak declension
in Ihren großen Hotelzimmern
("in your big hotel rooms")
aus diesen roten Autos
("out of these red cars")
zu allen wichtigen Autobahnen
("to all important motorways")
bei sämtlichen internationalen Gruppen
("with all international groups")
Mixed declension
nach meinen ersten Nächten
("after my first nights")
vor euren letzten Abenden
("before your final evenings")
mit keinen besonderen Wünschen
("with no particular wishes")
Strong declension
mit modernen Tagungseinrichtungen
("with modern conference facilities")
mit gepflegten Getränken und kulinarischen Genüssen
("with excellent drinks and culinary delights")
in zwei guten Studios
("in two good studios")
Notes
1. Both words meaning "all" - "alle" and "sämtliche" - take weak adjectival endings.