I've spent plenty of time driving mdt's over the years with both air brakes and hydraulic disc brakes. Yes, the air brakes stop better. No, you don't really need them at that weight. My trailer fully loaded behind my GMC C6500 stops just fine with disc brakes at 34000# total weight. Nothing against air brakes, everything else being equal including the price, I'd go for the air brakes. But not a deal breaker. And assuming a legitimately titled RV, legality of air brakes will not be an issue. Every "big rig" pusher diesel RV out there has air brakes, and you can drive it off the lot and all over the country with an ordinary drivers license and nobody will bother you.