[psomers]

While I don't disagree 100% with Iripguy, I wouldn't run my furnace with stair vent closed off. If you do make sure your carbon monoxide detector is working.
If your furnace is blowing 1000 CFM it has to be sucking 1000 CFM. With vent closed off it has to be sucking air in around basement doors and any other place it can get air.
My first 5th wheel had the furnace in living area under the refrigerator. It only had a vent hose running to tanks. Much more efficient.
On my Montana I have taken some rigid foam board and closed off my storage from the furnace area.
When your thermostat calls for heat the fan runs for several seconds before the burner lights and then a few more seconds before the heat exchange gets hot enough to produce warm air. Until that time cold air from under your camper is being mixed with cabin air.
My home furnace has two blowers. One for combustion and one for heat circulation. Much more efficient.