I used to find that camping was not always a great experience, specifically, I did not feel that great when I slept in a tent. This puzzled me for years, until I discovered that I was allergic to mold.
Once I figured out that my tents had a mold problem, the next step was to remove mold from the tent.
There are instructions on this website on how to get rid of mold from a tent. Some of the instructions really apply to smaller nylon tents that can be soaked in a bucket or laundry sink. There are also hints in that article that apply to any kind and size of tent.
The following is best done outside:
For large canvas tents, you can try putting the whole tent in a heavy plastic bag, and pouring buckets of water mixed with bleach into the bag. It is important that you mix the bleach into the water first before you add it to the bag, or you may get white patches on your tent where full-strength bleach contacts the fabric. Mix about 1/4c bleach to 20l water, then and the well-stirred mixture to the bag. Repeat this process until the tent is saturated with the water-bleach mixture. Tie off the top of the bag to keep the water from leaking out.
The next step is to empty the bag. Untie it or cut it open, and drain the water. To rinse the bleach water out of the tent, hang it from a line, and wash it down with a hose.
Allow the tent to dry completely before storing it.