I have a 2015 V3605 with similar symptoms. I can tell you about my rig – yours may be slightly different.
There are three large (3” or 4”) flexible ducts connected to the plenum at the rear of the forced air furnace in the underfloor space behind the rear basement wall. The shortest run is the one going to the bathroom – that’s why it gets so warm so quickly. There’s also a smaller duct run off that one that goes to the bedroom – with less than spectacular performance. The other two large ducts feed other areas of the coach through rectangular foam underfloor ducts.
If you remove the basement wall toward the rear of the unit, you’ll notice several feet of extra duct doubling back on itself and just piled up in the underfloor space. The first step in improving conditioned air flow is to remove the extra ducting and shorten the runs. The furnace plenum is easiest to access from the driver side of the basement. Disconnect the large ducts from the plenum, cut off the excess and reconnect them. Use a piece of sheet metal at the plenum to restrict the flow into the duct that supplies the bathroom – that will increase the flow into the others and keep the bathroom more comfortable.
Remove the floor registers inside your coach and seal the rectangular duct-to-floor connection with metal tape. That will prevent conditioned air from escaping into the uninsulated space above the plastic belly pan.
Remove the fireplace and you’ll see a small flexible duct (probably also way too long) running from the under-floor duct to a round register in the front garage wall. Shorten this duct as much as possible and consider replacing it with a larger duct if your garage use warrants it. While the area behind the fireplace is accessible, you may as well clean up the sound system wiring mess behind the television and the radio/cd player. Just look up while you’re on the floor working on the ducting.
Your return air system consists solely of the louvered panel on the short wall to the right of the kitchen range. The furnace draws the return air directly from the conditioned air in the kitchen area through this panel. The furnace shares the space beneath the bathroom floor with your water pump and interior plumbing connections. If you camp in an area experiencing a ‘hard’ freeze for several hours, leave your furnace thermostat set at 40 degrees or so to keep these pipes from freezing.
Since you have the basement wall open, you’ll also notice how much more basement space you can gain by moving the basement wall framing. My vacuum cleaner was near the center of the basement wall, so changing the bag meant crawling into the basement. There’s plenty of room under the stairs leading to the bedroom for the vacuum cleaner unit and in my case, I was able to cut out about six feet of vacuum hose that wasn’t needed. Check out archived threads for more info on these two mods.
Probably more information than you wanted, but I hope it’s helpful.
