Classic Winnebagos & Vintage RVs

So we took the the D20 Brave out for our first trip this weekend and noticed before we left that there was no gray water tank under the rig.  Did they come with a gray tank from the factory?  I've read folks on here swapping out gray and black tanks for a single large tank.  Is this recommended?  How do you keep the smell from coming back up through the piping  i??

There are traps on the sinks and tub, and the toilet has a flap that retracts when you flush it.  Unless the traps are dried out or the flap is leaking you shouldn't smell anything.