If your vent pipe has dropped far enough to touch bottom, it would be very obvious from the roof as there is only about 2 inches or so of exposed pipe between the bottom of the cap and the roof usually. If it has only dropped a little, it could get occluded by the liquid in a full tank. It's not very likely that it did drop but always possible.
Now as to the smell in the camper.... Even if you are not venting properly, you should not be smelling that smell in the camper if everything is right. Luckily, black tank plumbing is pretty simple. The most likely culprit is the foam seal between the toilet and the flange. They all get brittle and go bad but are very simple to replace. The next possibility is the seal on the flush mechanism on your toilet but if that is bad, you would notice that water is not staying in the bowl.
If it is not those simple things, then it would be a seal somewhere in the PVC pipes. The flange is screwed right onto a stretch of pipe that is usually straight and short and is in turn screwed into the black tank. The points at which the components are connected are below the floor and usually are fine.
The last possibility is that the vent pipe is leaking gas either where it connects to the black tank or along its length if it is not a single piece of pipe all the way to the roof. It is usually in a wall or behind trim and a cabinet.
It's almost always the foam seal though.
Current RV - 1999 Bounder 36s
Former trailers - '94 T-2770
'08 Keystone Sprinter 299BHSS