I've never added additional locks to any of the 12 RV's we've owned!
When you spray your lock with WD40. spray the key and insert it the lock, move it in and out several times. You should also insert the key slightly to "open" the keyway, then spray liberal amounts of WD into the keyway. If this fails to work you'll either need a new lock or a Locksmith. The issue with a new lock, of course, your same key probably won't work and you'll have an additional key to mess with. Good luck and please let us know how you make out.!!!!
Days camping (2016)----181 days
Days camping (2017) --- 82 days
2016 Wildcat MAXX 28RKX (33' TT), 2007 13' Scamp
2015 Ram Laramie Hemi, w/air suspension
30 years RV'ing
11 different RV's