Your inverter manual should tell what wire size to use based on length. Or you can use one of the free online wiring calculators. Be sure to account for the length of both wires. Normally inverters are connected directly to the battery. Existing wiring may be OK. Make sure the alternator and all wiring are adequate for the extra load. And be sure to account for headlights, AC fan etc. 30A might be to much for a TV/trailer with factory wiring.
I'd suggest you buy a $50 Sears DC clampon ammeter so you can determine the various loads. It will last a lifetime and be worth the price. Or borrow one.
There are many posts about running with the refer on LP which you can search.