The good news is that if youre using the trucks own Hella socket it will be fused. Any excess current will blow the fuse, so no danger. If the cable from the plug to the inverter is the same as that supplying the socket, again no worries.
24volt, 15amp giving 360watts sounds about right to me too. Inverters aren`t 100% efficient so if you limit all equipment connected to it to 300watts max all should be good.