You can't run the 45 amp converter at 45 amps from an 800w inverter. First the inverter will have a continuous rating of maybe 600w. The converter will draw in VoltAmps (VA) which is after counting the Power Factor it has of 0.7 so that 780w will be 1114 VA.
Not clear but that "30a" could be the converter's 120v spec as opposed to 50a like in a bigger RV.
As mentioned earlier you can do the thing when stationary so that the inverter is clamped to the car battery with short fat wires, hanging outside the engine compartment away from the heat, with the hood up and engine idling. But you must use an inverter that can handle the charger, and a charger that only pulls what the car can supply to the inverter while maintaining its voltage.
Doing this while driving means using fatter and longer wires routed somehow to get the inverter away from the engine heat. You can get the long 120v wire back to the trailer where the charger would be close to the RV battery though, if you rig it so it stays off the road.
What happens is when the car system is overloaded, the car's voltage sags. You can find what it can do by trial and error if you had a charger with amps settings, such as a Vector with 35, 20, 10 amp settings.
I was able to run at 35 amps of charger with a 1000w MSW inverter with the 2003 Chev truck idling, but higher amps caused the truck's voltage to fall off. Don't forget to turn off the car's blower etc, which uses some that you want for the inverter.
I also can run a Vector at the 20 amp setting with a 400w MSW inverter that runs at 320w.
