You can tow motorcycles behind a travel trailer by buying a motorcycle trailer and attaching it to your travel trailer. This is called triple towing and is legal in many states. However, check the laws of any states you plan to drive through, because some do not allow it.
Since most travel trailers do not have hitches installed, you probably need to have one put in before you can tow motorcycles. You can generally have this done by a local welding shop. Depending on the setup of your travel trailer and how many motorcycles you plan to bring, you may be able to have them install a motorcycle rack directly onto the back of the trailer. This may be easier to drive and help you avoid getting in trouble for triple towing in places that do not allow it.
If you choose to pull a second trailer, make sure you check all the details of local towing laws. Even states that allow it often have restrictions on length, weight and other factors. Some also have restrictions on what type of second trailer you can tow.
You also need to choose your towing equipment carefully. Triple towing rigs are prone to swaying, so stabilizing hitches and other safety mechanisms are important.