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.
Dan-Retired California Firefighter/EMT
and Zoe the Wonder Dog(R.I.P.)
2016 PrimeTime TracerAIR 255, pushing a 2014 Ford F150 SCREW XTR 4x4 3.5 Ecoboost w/Max Tow Package