It would depend on your usage. If your rig sits more than you drive it, like less than 3,000 miles per year, I would use Quaker State 5w-30 conventional. You should change the oil once per year regardless if its driven or not, conventional oil these days is good oil, cheaper than synthetic if the unit sits. RV motors are magnets for moisture when they sit in shaded, damp campgrounds. If your like me and hammer the daylights out of the motor, A/C running, stop and go traffic, running flat out on the hot summer days then 100% recommend Mobil 1 5w-30 in your gas motor. Had a class C and those motors / trans run very hot.
East Bound & Down- We Love Camping :)