Don't forget your slide, awning, water pump, LP/CO detectors, radio, stabilizers (if you have them), electric jack (if you have one), any powered meters, hood fan, breakaway switch, etc. Some of that you can use your TV to run, but was just trying to illustrate the mirad of little things that are easy to forget, but can add up.
Most rigs will have around a 1-2amp "phantom" draw, which you will get no return from. So with your automotive battery (without knowing the exact specs) I'm pretty confident you only have 50-85 amp hours of capacity. Most certainly not much over 105ah tops. So with a parasitic draw... in a 24hr period.. that will leave you, conservatively, 0-25ah "available" (50% SOC) without having turned on a single DC device.
With your automotive battery charger... that's fine for the auto battery you have, however, you will more than likely be in a near constant discharge/charge situation. Not a great place to be for a healthy battery and a hassle free trip.
I think I get what you're trying to do.. you have a battery and charger already, so out of pocket at the moment would be minimal if you nixed the converter. Is that about right?
Just to illustrate. Your 12v power resources at the moment... A battery, an auto charger, no converter, and shore power. You saved the $150 by nixing the converter.
A little down the road... No converter, battery is shot, an auto charger that's not really usable if you go with a deep cycle battery next time, and shore power. In other words.. no means to run the 12v side of your rig unless you fork up for new batteries ($170) and a compatible converter or another charger ($150).
I think what everyone has been suggesting (without the "long story") is that you are better off to pony up the $150 for the big beautiful converter, have unlimited 12v available now and onward, and a charger that will be compatible with true deep cycles when your forced to replace your battery (which you don't have to worry about for awhile since you already have unlimited 12v).
It puts you ahead of the game now and later with the least amount of hassle.
That being said, and if money's a little tight right now, a fella's gotta do what a fella's gotta do to go camping.
I think most of us have run on a jerry rig more than once. It's entirely up to you and hope you didn't feel too pressured. Just for those that have been around the block had in the back of their minds, I'm sure, that you would be getting "more" for "less" going the converter route now.