When did the Roman Army decline?

[Flavivs Aetivs]

Chalons? Yes that estimate would be accurate, Aetius fielded about 22,500 men at the Battle of the Catalaunian Plains, most of them Veteran Pseudocomitatenses with a core force of about 5000-10000 Bucellarii and Auxilia Palatina (Elite Crack Troops). All of them had been equipped with Armor and were well supplied, as evidenced by Sidonius Apollinaris' letters to Tonatius Ferreolus. In those letters he also mentions that there were enough supplies leftover to equip (some of) and feed the 40,000-ish Barbarian Allies Aetius had recruited, who by this point were of the same quality as the regular Roman soldiers. It took them about 2 years to prepare for this invasion, and there was also a famine in Italy at the time.

However, your view of the army is highly misguided.

Comitatus may enlighten you:
http://www.comitatus.net/armytroops.html