There are 2 ways of installing a battery disconnect switch. The first way and probably the most popular is connecting the disconnect switch between the batteries negative terminal and the TT's frame ground. The second way is to insert the disconnect switch between the positive terminal and the TT load connection. Oh, and the cable that connects the 2 batteries together should be the same size or large (AWG) then the thickest cable connected to the battery. If you add a high output inverter, the mfr will specify the needed cable size to use.
I included a drawing below, any questions just ask.
Good Luck, the negative installation should only take about a 1/2 hour with parts.
Don
My Registry
RVing with SOLAR