I have no idea why they decided to make the fifth declension last, because it is so easy. It is just as easy as the first declension if not easier. Every fifth declension noun except "dies" is feminine. The fifth declension thematic vowel, "e", is kept unchanged in every case form. The nominative singular is always "-es", unlike the third declension where the nominative singular can be anything and you have to memorize it for every noun.
Here's a paradigm for "res" ("thing", "affair"), which also works for every other fifth declension noun:
case singular plural
nominative res res
accusative rem res
genitive rei rerum
dative rei rebus
ablative re rebus
There are no fifth declension adjectives, and there is no fifth declension in Greek.