50A to 30A dogbone
The answer is yes... It will work and seem OK.
There is some risk in doing this. Here is the long answer...
You are plugging into a power source that can provide 50A of current to your trailer.
If your trailer tries to draw more than 30A, then you are counting on your 30A Circuit Breaker(CB) to trip and protect the wiring inside your trailer.
This is why the 30A is installed for you main trailer power in. That's it's job.
Here is where the risk comes in...
The trailer 30A CB only protects the wiring from the trailers 30A CB to the internal wiring of the trailer... It does not protect the wiring from the power pole up to the trailer 30A CB. (Your hookup cord/plugs)
The wiring in your trailer and your hookup cord is sized for 30A.
50A flowing through wire sized for 30A will get hot and eventually could cause a burn point (usually an open circuit but could cause a short too), and it could cause a fire.
When I designed systems we had to design them so that no single point failure could cause a fire or burning. These are UL, IEC, CE and bunch of other electrical standards requirements.
HOOKED UP TO 30A SERVICE:
When you are connecting to a 30A power source service, it is supposed to be protected with a 30A CB at the source (power pole). Then you have another 30A CB in line in your trailer. So both 30A CB's would have to fail simultaneously. This is almost impossible.
Fail means that the CB does work like it is supposed to and does not trip at over current >30A. Thus.. You meet the requirement that no single point failure could cause a fire.
HOOKED UP TO 50A SERVICE:
When you connect to a 50A service then it is able to provide 50A without tripping the service's CB. Now you are counting on only your trailer's 30A CB to work correctly. They almost always do and it is very rare for a CB to fail by not tripping when it is supposed to. They almost always fail by tripping before they are supposed to at a lower than rated current. They are designed to be "failsafe" this way. But it could happen...
For your to have an issue several conditions would have to exist.
1. You plug in/turn on loads in your trailer exceeding 30A. If you don't ever do this than your risk is almost zero. But Air Conditioning, Microwave, toaster oven and hair dryer will easily do this.
2. There is a short somewhere in the power lines in your trailer AND your 30A CB fails. This is rare and unlikely, but it can happen.
3. If the short is between the dogbone and the trailer's 30A CB, then your power hookup will still be protected to 50A limit. Shorts usually draw very high current and the 50A service CB would trip right away.
Also, shorts are unlikely unless you are careless and have cut or frayed insulation in your power cord or abrasion point in the trailer wiring. You should inspect these regularly.
So.. Myself, I would use the 50A male to 30A female dogbone adapter without worry. But as an electrical engineer, I could never recommend this as a standard practice due the the rare and unlikely set of conditions that could occur and cause a failure.... fire!
Sorry for the convoluted answer. I am sure others will jump in and say to never do this. But the probability is extremely low...
You have a much higher risk of getting into a accident driving to your destination than these conditions ever occurring simultaneously.