What amp fuse for charging circuit from truck Alt?

Yes it was just the absent fuse, now with a 40A fuse in the correct spot the camper batt is seeing 13.8V with the truck running. I'm still going to hook up an isolator but will connect it using the stock wire that goes through the plug at the bumper. Maybe a solar pnl someday as well, but the sun is pretty fickle in these parts and I do have a 1000w Honda Gen but it is nice to start out the evening with a full charge after a days drive.