Many motorhomes end up overloaded simply because people put too many things into them. There is no quick and easy way to determine the weight of the items you have loaded, so there is a temptation to keep loading stuff until every available space is filled. People are loading by volume, but weight is far more important.
Unfortunately, you may have exceeded the load-carrying capacity of the tires and/or axles long before everything is stowed. The only way to tell if your motorhome is overloaded is to have it weighed. Follow the steps in this how-to article and you will be well on your way (no pun intended) to safe and worry-free RVing.