converter to battery wire length ?

I read that the distance from the converter to the battery should not be more than ten feet the wire on mine is 8 gauge but is closer to thirty feet. my trailer is a 32' park model that is total electric it has 12 12 volt lights with 2 bulbs each and a 12 volt thermostat all appliances are 115 vac there is no convenient place to put it closer. do I need to use larger wire? or will 8 awg be big enough. I have a new converter ordered and would like to do all that needs to be done at the same time. if the wire is not big enough what size do I need?