What kind of distance sensor are optimal to measure distances to surfaces at an angle to the sensor?
Using ultrasonic sensors would be a very sensible solution in your use case.
I am currently working on a similar project with a company in Denmark, where the objective is to detect cars on parking spaces. For this task, I used two of these ultrasonic sensors from DFRobot for the prototype:
https://www.dfrobot.com/product-1935.html
They were very easy and simple to setup on an Arduino UNO and worked perfectly fine even when two were sensing at once.
Though I have to mention, that they need a serial connection each, but this can be worked around using a multiplexer or a developer board with more serial connections.
I also found that the ultrasonic sensors work better than time of flight sensors in outdoor conditions. They are very robust and wont be as affected by dirt for instance.
A little speck on the lens of a ToF sensor and it will be rendered useless.
Ultrasonic sensors are also widely used in the car industry for parking aid solutions where they have to detect lots of different surfaces and be very precise in doing so.
Ultrasonic sensors are great. And yes, they can interfere with each other, but as long as you dont put them on top of one another, it should not be an issue.
You should be able to detect the distance to a trailer in two different places using ultrasonic sensors as long as the trailer is within the sensors range.