Late Roman Army during the 5th century

[Renatus]

I am sure I have read in my collection that those who enrolled in the auxilia units were not expected to perform the non-combat roles that the Legiones were expected to perform i.e. building camps and roads.

You may be thinking of Ammianus 18.2.6. When Julian was restoring the defences of a number of cities on the Rhine in AD359, '. . . the auxiliary soldiers (auxiliarii milites), who always disdain such tasks, induced to diligent compliance by Julian's fair words, willingly carried on their shoulders timbers fifty feet or more in length, and in the work of building rendered the greatest service' (Loeb translation).