Go with as large of and as many batteries as you can hold and afford. The axiom "You can never have too much power." holds true for off-grid RV'ing :)
To your questions: HEATING elements are resistive loads. They consume their rated wattage from power on to power off, there is no surge or startup current.
Anything with a motor is an inductive load. That's your rooftop AC, your microwave (it has a motor and huge transformer), the bedside fan, etc. They have startup current and run current. The startup current for something like a A/C unit is huge, a microwave's not so bad. Figure 50% for a microwave and 100% for an A/C (1200w microwave = 1800 watt surge, 1800 watt A/C = 3600 watt surge)
If you have a big budget, consider a 48 volt system (8x 12v batteries in two banks for example). 48 volts will let you use smaller wire (maybe 00 gauge instead of doubled up 0000 gauge). you can run dual high-wattage inverters, so you can power up both legs of your RV's 50 amp service.
