Quiet fan for furnace vents?

My 5th wheel is 23.5', so I have to be stingy with space. I did not get a optional fan as it was a Procomm Ice house heater. And, actually I want this fan to blow the warm air from the living space down into the basement to add some warmth down there.

Computer case fans. I use them in place of a Fantastic Fan. Quiet and very inexpensive.

I am only looking for something to send a little bit of warmth into the basement. I don't expect much output from the floor regulators (most likely all but the farthest one will be capped off as I expect the blue flame to keep me PLENTY warm, but just want SOME heat under the living space for now.

Not sure what type you are looking for but they have many different ones here.
http://www.megagrowers.com/

Are you looking for a fan for just the duct to the wet bay, or one to circulate air throughout the existing duct system?

If it's the whole system, a few quiet muffin fans aren't going to produce a lot of airflow. For that case I'd look for something more along the lines of a squirrel cage blower—maybe some automotive vent fan would not be too hard to adapt, and should be reasonably quiet once buried in the enclosure where the furnace was. For maximum efficiency, don't use the speed control resistor but run it at full speed (assuming you have that sort of a speed control—I think a fair few newer cars use an electronic speed control for the fan and consequently can offer more than three or four fan speeds).

If it's just that particular duct, a muffin sort of fan might work reasonably well. Generally speaking, the larger the diameter, the quieter...though of course a high airflow fan is noisier than a lower airflow one. I suspect there's not enough of a difference in energy consumption between equivalently specified fans to worry about. Dual ball bearing models may be more long-lived than sleeve bearing ones, though a good quality sleeve bearing can last a very long time. (Not all sleeve bearings are good quality, though.)