Solar Panel Motorhome

I'm worried that the leisure battery is overloaded if both solar and mains are provided at the same time.

No. The solar panels often can not provide much in the way of power. They can keep the battery from your vehicle to unload if it is too long, but can not overcharge a battery.

No you do not disconnect them. In fact, this is not good for your battery, your concern about the charge was not necessary because there should be a safety in the construction of this