Hello Forbes,
It is UK and EU law that a trailer must exert a downward force on the tow cars hitch. It must be a minimum or 25Kg. The maximum is determined by other factors and specifications which I will discuss later.
You will find that all caravans will have more in front of the wheels than behind. This is necessary to generate the nose load which in turn is necessary to aide towing stability.
You describe how the mud flaps of you car are almost scraping the ground when you have a caravan attached, which implies that the suspension of you car is sinking lower than you would like or expect.
You can and should resolve whether the outfit is loaded properly and then if the there is a problem with the suspension or mud flaps.
Firstly it is important to check the nose load of your caravan. As mentioned above the minimum nose load is 25Kg. The maximum has to be calculated by using data from the car, tow hitch and caravan specifications. Each item will have been given a maximum permitted value by its respective manufacturer. If there is any difference between the values, you MUST use the lowest value as your upper limit.
You now need to measure the nose load actually exerted by your caravan. This must be done on level ground. Attach the caravan to the car and raise all the steadies and jockey wheel off the ground.
Measure the height of the tow hitch to the centre of the ball.
Unhitch the caravan, and arrange the load measuring device to support the the hitch at exactly the same height as if it were hitched to the car. The nose load should be measured with the jockey wheel and all the steadies raised above the ground.
You now have two bits of information: height and load. If the nose load exceeds the limit you have gleaned from the car, tow hitch and caravan specifications, then of course you must reduce the load by removing or repositioning the items inside the caravan.
Once the load is compliant you need to check the height of the ball when the caravan is again hitched to the car.
The EU specify an acceptable range for the hitch height as the centre of the ball must be between 350 to 420mm above the ground. If you hitch is outside of those limits, then you should have the suspension of your car cheeked. If the hitch is within limits with a compliant load then it is just a feature of the depth of the mud flaps.