I have the sister to your's, a 2002 Itasca Suncruiser 35U. As near as I can recall, the gas furnace returns are under the bedroom vanity and in the wall next to the bathroom door. The basement air system is the only thing that uses the return under the bed.
Despite the issues I've had to deal with on mine, I think the 35U is one of the best motorhomes that Winnebago has produced. The floor plan is great.
Feel free to PM me if you want to communicate directly. I've pretty much been through everything. A quick search for postings under "BobC" will show you what I've had to deal with. When searching, the Google search box at the top of the forum pages is the best way to search.
One thing you'll definitely want to check out is the condition of the fittings on all your slide hydraulic hoses for leakage where they're swaged onto the hoses. HWH, the maker of the slides had a problem with their hoses in the early 2000s. If you find any signs, it's best to just replace all the hoses to the unit in question, it will be cheaper in the long run. You can order replacements from HWH by the number on the hose labels. Length is critical. Hopefully your labels look nice and new meaning you hoses have also been replaced.
In addition, read up on Winnebago windshield leaks, another potential trouble spot that you should deal with asap if you have any drips.
You can create a signature that lists your year and model like mine below, so it shows on all your posts.
2002 Itasca Suncruiser 35U