RV Repair: Adding a battery to your tent trailer, deep cycle battery, prong plug

Question
I purchased a 1971 Starcraft Starmaster 8 tent trailer. Someone added an aftermarket deep cycle battery that is connected to a dome light, a 12 volt cigarette-style power outlet, and nothing else. During a roof replacement, I added a 12 volt vent w/fan. I plan to run the wires to the dome light to get power.

I'd like to connect the battery to my car through the pigtail, add an isolator so my car battery won't drain, and then I'll have a self-charging system. My trailer has a 4 wire, 4 prong plug and my car has 4 wires going into a square 7-hole plug. Something obviously is going to need to be adapted or replaced. Someone told me that if you're going to be charging a trailer battery with your car, you should use a round plug, because the wire that charges your battery is heavier gauge.

I know enough about wiring to properly install a car stereo, and to follow the schematic to properly wire that fan to the dome light, but not much more. How should I tackle this? I'd like to do as much myself as I can. Thanks!

Answer
Yes you will most likely want to replace the plug and receptacle with a 7 wire round this way you will be ready for any RV, Wal-Mart has these for $24. Wire the aux for the trailer and follow the schematic and you should have everything you need. Send me an email and I will send you a schematic for the plugs. gnorman46@sprintpcs.com