Shottky diode, cheaply obtained along with a heatsink as well from a deceased PC power supply. Use the one on the 5V rail, and connect the 2 outer leads together and the centre to the other wire. Outer leads to the solar charged battery and the centre to the car battery, preferably using relatively thick cable ( 2.5mm minimum, 4mm better) and a 10A automotive blade fuse in line as well. The batteries need to have a common negative connection, provided by a short cable joining the camper batteries negative terminal to the chassis if there is not already one. You can also add a 20A rated switch to disconnect the circuit if not needed, simplest is to use a 20A MCB as it then can double as both the disconnect and a fuse.