Hmmm, something is amiss here, and I think I have an idea.
You mean that you found the vent on the roof? One or two? The "drain vent" can be the tank vent if it splits from the drain line below the p-trap. No, I don't think that this is a venting issue. Like McKannick mentioned, the air would find a way out. If you want to try again, hold the toilet flush valve open and let it vent out of the toilet hole.
Did you ever figure out where your bathroom sink is draining to, the black or the gray?
I think your tanks are full. I find it curious that when I was looking up specs for your rig, on the same chart, the smallest model in the class, the 19MTT, specs a 28 gallon black tank and a 35 gallon gray tank. It would not surprise me at all if they installed the wrong size tanks on your unit when building it! That is worth checking out, but to do so you'll have to climb underneath and inspect your tanks. Find any markings if you can... capacity or at least a model number or something. If not, then measure them with a tape measure. Then, do the math... multiply the height x width x length of the tank in inches to get the total cubic inches. Then multiply that number by 0.004329 to get US liquid gallons.
If the tanks are different than those listed specs for your unit, then you can call Gulf Stream to raise hell with good data.
Good luck, and please let us know what you find out.
p.s. I also "assumed" when I got my rig, but I found my toilet and bathroom sink go into the black tank and the kitchen sink and shower go into the gray.
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