When hunting with a muzzleloader, I am often tempted to keep the powder and projectile (tripple seven & powerbelt) in the gun, especially if I am going to hunt for several days.  I am not sure though if the powder will diteriate from condensation to the point where it is not effective.  Also, how corossive it will be to the barrel.  Any advice would be greatly appreciated.
Snowshoe MA

That is a major concern. There are people that leave a rifle loaded for weeks, even months. As long as the rifle was not fired, it will not rust from fresh powder. I personally do not like to leave a rifle loaded. I had some wake up calls doing that. Now granted my garage in the winter is an ice box and very humid. That might have lead to the rifle charge being contaminated. I am going to try this year leaving the rifle in the wood room just for testing. I personally shoot mine off each day at the end of the day, swab the barrel clean and reload it the next morning.

If you have time before season, try this. Load the rifle and store it where you would normally store a loaded rifle during hunting season (if possible). Keep the rifle in the same temperatures as you hunt in. So if it is 50 outside, store is somewhere it is about the same. Leave it there five days, loaded.. then take it out and fire it. See if it goes off, and if it hits the same spot.

I live in Wisconsin where in the winter muzzle loader season we have lots of snow and very cold temperatures. So I have choices of the garage, the wood room or the wood shop to store in. I have had rifles store loading fine. BUT I have had two of them fail to fire the next day from being stored in there. So I like to know, my rifle will go off.

Welcom to the BB!  Unfired powder is not corrossive so it can sit there and not corrode the bore for many years.  Rapid temperature changes do cause metal to sweat as it heats, but not as it cools, so you can dmaage the charge or even kill it if the barrel is allowed to warm and cool many times.  Air humidity can also eventually affect the powder.  As for black powder, I have shot some loads that had been loaded for about 100 years.  It never degrades in power so long as it is dry when ignited.

I use loose 777 and leave mine out in the cold with the cap off and have gone many days and one time for six weeks or more and never had a problem with mine not firing!

leave mine loaded for the whole season never had a problem

I leave mine loaded all through the season and even through rain and snow I have not had a problem with ignition.

I use only real bp. Living in PA we can get big swings in temps and humidity during the fall and winter. I do leave my rifle(s) loaded for at least several days if there has been no rain, high humidity, or temp extremes. I do not subject a loaded rifle to warm temps if at all possible. I once forgot a loaded flintlock for about 3 months and fired it off in April. It appeared to be just as strong and as quick or possibly even quicker ignition than a just loaded rifle. If I fire a rifle then it gets fired and cleaned as soon as possible. I did have a load that was very difficult to ignite after only several hours of being carried in wet humid conditions. I have always adhered to "If in doubt shoot it out" and clean and reload. In a flintlock I have had the pan powder go wet and clumpy after only minutes.

I load mine at the start of the season and leave it loaded. I too have never had an issue with it not firing.

I'm also from PA. and play it by ear with my flint lock , but usually don't have to shoot it , I gauge it on how the pan powder is reacting . My in-line I don't worry about .
Read an article a while back about a guy that would throw them in a water troff for cattle for a given time then shoot them . The ones that fired were cleaned , re-loaded and back in the dunk for a longer period . This time increased until one was left . I was surprised that it was a Knight Wolverine , same as the one in my cabinet .

Remember the story back a year or so where the guy found a muzzleloader in a dumpster? He was heating the breech plug to free it up when it went off and blew his kids head off. no telling how long that sucker had powder and ball in it.

Lets think about safety. Leaving a loaded rifle around that cannot be easily checked is an accident waiting to happen. Modern BP are so easy to clean why wouldn't you empty the gun at the end of the day. Beside now they even make a tool that hooks up to a CO2 carteridge so you can blow the powder and bullet out of the barrel without actualy firing the weapon. Lets think safet first.

I use a flintlock, and I leave it loaded all season. I plug the touch hole with a feather, and store it out in the cold. Never had a problem

Problem with unloading" muzzleloaders is it ruins a projectile, oftne expenssive ones.  They cannot be reused with any expectation of the desired accuracy because the fit has been compromised.  Safety is no real concern so long as you are educated in muzzleloaders.  De-capping and de-priming/plugging is completely safe - actually it is virtually the same condition as having cartridge ammunition lying around.  All you are doing when loading a front-stuffer is handloading a giagntic metallic cartridge.  Checking any gun upon picking it up to see whether or not it is loaded is always our job-1 and it is simple to drop the rammer down.  A metallic bounce means it struck breech.  A dull thud and little or no bounce means it likely struck a projectile or bad fouling.  For added safety, simply insert a little flag intot he muzzle, or tape a note to it saying "LOADED!" and that ought to tell even idiots to be careful.

There's more than simple truth to what Shiloh points out and is something to consider....
IF something terrible happens to you, will those who find your firearm know it is loaded and how to handle it. For many of us, this lifestyle is a family affair and not an issue, but worth thinking about otherwise....if a meteor hits my bed mid-season, my son knows what to do, but that was not true 10 years ago....