Even IF you were to find a 'manual' it would be VERY generic.......
Really want to get to know your RV.
Take pen/paper n hand and write down each BRAND/MODEL of appliance, component etc
Then goggle that BRAND MODEL Service Manual..download/print and place in a 3 ring binder.
NOW you will have a 'manual' that is specific to your items.
Otherwise.......it is a box on wheels with basically the same stuff as every other trailer
AND........which model is yours?
9 different models of 5th wheels in 1991
1991 Aluma-Lite Travel Trailer/5th Wheel Prices, Values & Specs - NADAguides