I don't see any point in disconnecting the batteries if there is a working solar system otherwise available. If you are going to disconnect, you only need to remove one cable, assuming this is a normal dual-battery installation consisting of two 6-volt batteries. They are connected in series so the same current flows through each battery and cable: disconnect at either terminal of either battery and current flow is completely interrupted.
If you are going to touch battery terminals to disconnect cables then it is inherently safer to disconnect the negative cable than either the positive cable or the connection between the batteries, due to reduced risk of accidentally making contact between points at different voltages and getting a shock.
If you plan on regularly disconnecting batteries, it would be more convenient and safer to install a switch for that purpose, or the style of battery terminal hardware that comes apart by turning a knob (no wrench required). I would probably use one of these: Keyed Battery Master Cut-Off Switch
Only $8, available under various brands at many stores of many kinds (hardware, automotive, RV...) handles all the current required, could be mounted at the battery or through the battery box or in the interior of the trailer, and worked fine in my race car. Fancier versions are readily available.