Should be on both axles, best way is to look behind the tire wires will be seen going to both backing plates.
There is IMHO no normal when it comes to RV's because it depends on how much you travel and how often you use it.
One could go years with short trips or a seasonal site before brake replacement is needed.
2016 F350 6.7L LB CC Reese 28K 2014 Chaparral Lite 266sab
"Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety." 2014 19 days camping 2015 17 days camping201620 days camping