Weatherproofing Canvas?

Yes, Blue Coral Wash and Wax. It has 10% bees wax that after cleaning sticks to the openings in the weave. It's at Walmart for around $4 in the automotive area. It also cleans rubber roofs and sides of RV's when mixed in a bucket with Clorox about 3 cups of each to create an emulsion that is white as the Clorox bleaches the Blue Coral, which retains its wax that after rinse adheres to surfaces. Then just Blue coral every wash builds the wax up and eliminates need for waxing with polishes. Just wear white when using Clorox and don't get it on vegetation as it turns to salt when water is added and it dries. Rinse well off RV and lawns.

Are you sure what you are feeling isnt condensation forming on the inside, not leakage coming from the outside? You mention that it started happening later in the season which leads me to think outside temps have probably dropped leading to increased condensation.

+1 on checking for condensation and a leak. How old is you HTT?

There's no substitute for a wax polish on a hard surface, NuFinish is highly rated by Consumer Reports. Blue coral doesn't leave a wax finish worth a flat beer, and washing with it won't remove oxidation like a regular polish. I suppose it's fine on soft finishes, but I would never use bleach on canvas due to its potential to damage the canvas, many owners manuals advise against its use and recommend a mildewcide like the one below. Blue Coral is a good route between waxes, certainly on hard finishes, as it won't strip the old wax and helps minimize waterspotting if you have hard water like we do. For the canvas portion of your tentcamper, it's CanVak of course, available at Cabela's.

My Starcraft is a year older then yours and started doing the same 2-3 years ago. When it rained heavy I could see the lower portions of the zip up windows were wet. I first used Seam Grip on all the seams. once that dried I used Trek 7 to spray the zip up window covers both inside and out, a couple times. The zip up part seems to be a lighter type of material. I've had no issues since.
When I sprayed, I tried to be careful not to get the spray on anything but the window covers as I didn't know if it would have an adverse reaction the the Aqualon sides and roof.

If you are interested, this from a custom canvas shop, Nelis: Aqualon

Aqualon was previously sold somewhat frequently by Nelis Customs to offer a fabric similar to a popular Aquaknot branded fabric sold by another shop in Pittsbrugh, hoevever it has been all but phased out of our lineup of recommended fabrics at this point due to their being what we consider better alternatives that are equally affordable on the market. While we are phasing it out and recommending better alternatives, it does have its uses and we will continue to offer it as long as it continues to have a 5 year manufacturers warranty because it is an affordable and somewhat durable vinyl coated polyester marine fabric that weighs just 9 oz. per square yard which can come in handy. Aqualon offers a manufacturer's 5 year limited warranty to not crack, peel, harden, discolor, or flake. We caution using Aqualon and feel there are better products on the market because its coating is only applied to the top side and seems to be more fragile these days, but if you are looking for the absolute cheapest material Nelis Customs will use - this is it. The issue with Aqualon in our opinion is the durability of the vinyl coating which we have found to be easy to scratch and damage when the cover is pulled across something other than a smooth surface when installing on the boat. Nelis Customs plans to phase out Aqualon in 2010 and replace it with a more durable, equally priced, alternative called Harbor Time.

you shouldn't need to weatherproof the canvas.
your issue is more likely either condensation or leaking through the seams. either can be easily dealt with.

Thanks everyone. It's definitely not condensation on the inside. It only happens when it rains and we have taken precautions (such as opening the windows at night) to avoid condensation when the furnace is on. Padlin, your issue sounds exactly like what we are dealing with. I will definitely get the Seam Grip and Trek 7.

Having used Seam Grip on backpacking tents, I found it sticks to itself when you fold up the tenting, not permanently but it makes it a hassle to open. To get around it, after you complete the job and everything dries, lightly powder the Seam Grip. I just used my finger dipped in powder. Only takes once and you're good to go.