Is it a class A or class B or C? How long and how heavy? What will it weigh going down the road and will you tow with it?
I think the last year Dodge put 318-3 motors in motorhome chassis was 1970, is yours one of those?
I know several SO CA drag racers that had mid 1970s class B and C motorhomes with both 440 and 360 in them and they both like the 360 motors better shruggy

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)