There is a duct on the bottom of my furnace and it says not to block that it can cause an explosion I would like to know if I can make it into a cold air intake for my house trailer seeing how there isn't any cold air returns and it cost me a little more than 400.00 dollars a month for propane and that is all I use propane for any advice would be much appreciated thank you

I am not a furnace expert but I can tell you that blocking up that vent is definitely not recommended. What I would suggest that you do if your propane bill is that out of whack - have your furnace looked at as it sounds like it is in need of recalibration as you are using so much propane. You didn't say where you lived - i.e. what state.

Mobile homes are notoriously drafty and leak warm air like a seive. What kind of windows do you have also? Jealousies or slide up and down like a regular house. You did not indicate the age of home either as some older homes don't have enough insulation in them to really do much good in cold weather.

With propane furnaces it is always best to call in a furnace guy because as you said they could explode. Don't guess on this as it could very well be the wrong guess.

Leave the duct near the bottom of the furnace alone. It is there due to a HUD safety regulation. It is combustion air which is necessary.

Seal the rest of your trailer up to prevent high utility bills. Windows and doors are big offenders as well as lack of insulation especially in older units.

The new mobile homes are not to bad on utilities.