Your idea is practical and I would suggest useing 10ga wire. Applied to low voltage dc current,all 10ga wire is not equal. The more strands in the 10ga conductor the better to carry current without heating and subsequently leading to increased voltage drop. Insert a 30 amp inline fuse or circuit breaker as near truck battery and alternator as practical. As has been suggested,you can go through 7 way prefably center #7 pin or use #4 if #7 is in use already. A word of caution if you tow other trailers or tow this trailer with other trucks. #7 is normaly reserved for back-up lights on trailer. #4 is for 12 volt and may be in use so if you use #4 and it's less than 10ga,confirm that #4 is 12 volt to trailer battery then change to 10ga conductor from truck. Make certain #4 from 7 way to trailer battery is 10ga or larger. Unless there is an isolator inline or 7 way unplugged while useing trailer battery,the truck battery will be discharged at same rate as trailer's. Confirm that #1 is white ground at least 10ga. If #1 is smaller than 10ga,add ( double lug )a 10ga on both sides. Route truck side to nearest place on chassis it can be well grounded to. Attach trailer side to close by trailer frame then continue a 10ga to trailer battery negitive cable. This arrangement will carry up to 30 amps to charge battery. While it is true that if trailer battery is near discharged and truck battery is full charged,the amp flow to trailer battery will only be 15 to 20 amps and drop as battery charges up,that's not nessarly a bad thing. If the alternator could "see"only the dead battery it would deliver 50 to 100 amps then blow the 30 amp fuse before melting wire.(Bet some were wondering why I suggested a fuse). If you don't normaly drive long enough to bring trailer to full charge,you can switch fuse to fusable link and increase wire size to accomindate alternators full output along with a battery switch that lets alternator "see"only trailer and fast charge when you choose. The wire you need is welding lead that will cost some coin which makes a gen set look like better option.
