I recently learned that a Lithium battery can accept power at greater than 100 amps. So the RV in question had a limiting device on it to restrict the amperage going into the Lithium battery pack, so not to melt the alternator. In your case, you might have a 20 amp circuit breaker feeding the camper +12 circuit, and that fuse would quickly trip if the truck is 14 volts and the battery 12.5. A auto reset circuit breaker might help, or run a second power line that is sized at 30 amps and a auto reset circuit breaker that size?
With 500 watts of solar, that is about 2,500 watts per day. So a 100 AH battery at 12 volts is 1,200 watts. 2 batteries would seem enough to save all of 1 day power, and then some. I used to have a 400 watt solar system on a 97 Bounder and 400 AH of 12 volt batteries. Of course that battery should only be depleted by 50%, something the lithium does not care about. I was never without power, especially after switching from the tube TV to a flat screen that used much less power.
Personally I think you will be fine with only 200 AH, and being so expensive, buying more battery than you will ever use is just expensive.
