a manual would have given you no info on the size of the tank, nor anything else about your Salem, since all you get is a generic RV manual.
try RVWeb.com and see if they have the specs. it has the most info on older RV's.
and Zoe the Wonder Dog(R.I.P.)
'07 Roo 23SS pushing an '07 Chevy Avalanche
Equalizer WDH and Prodigy BC