Most converters only sypply a small amount of current to charge the battery. That is usually about 3 amphers. That is fone of the battery is fully charged. If it is discharged more than 10 per cent use an external charger to top- it off, then the converter should keep it up.
I have no knowledge of 8 pin wireing. You need to use a meter to figure out the layout. Mark whe wires and look up the 7 pin configuration and attach the wires. I would guess that the stop and turn circuits are split. good luck.
Fivers can be stayed in anytime they are parked. In some states they can even be rode in while traveling. If they are set up correctly they will be level while going down the road. It just a matter of having the hitch installed correctly. We have been using one for 22 years.