My Atwood manual states my furnacce draws 3.4 amps, the Jayco manual says 12. ... huge difference when trying to find the right battery. So I put an ammeter in the circuit. The 3.4 is correct.
An electrician I know told me DC doesn't have much voltage drop, so the ratings published for the different devices are probably close. That turned out to be true.
As you have seen so many times in this forum, incandesant lights have a big draw - about 300% compared to LED. The water pump also draws a lot of current.
An interesting note was the constant (parasitic) draw - those items that run "all the time". Your refrigerator controls, gas detector, and clock, together use about 1 amp. In a 24 hour period, that's 25% of the total battery capacity of the one that's pictured. Or 50%, if you only want to discharge to half capacity. So for a smaller 12 volt battery you can use half the recommended charge in a day without turning anything on. Pulling the inline fuse, or installing a disconnect switch, is a good idea. That's next.