Yes you can tap into the transfer switch, that is how most inverters are installed in Class As, the only issue is your inverter is too small to do that should really have it only powering a couple of outlets. Also I believe the recommended installation is to NOT have them in the same compartment as the battery and the battery fumes combined with the electronics could cause and explosion or the fumes could cause the inverter to go bad. The ground is recommended but only if your trailer is not using the frame as a common negative circuit.