If you are powering something like a laptop, it can change the wattage as the screen gets brighter or dim, so wattage can change minute by minute. Still for battery budgeting reasons, consider it about 1.5 amps per hour, and you will be safe.
The inverter input is about 16 watts with a 14 watt load. If the engine is running, then the input voltage will be about 14 volts, so about 1.1 amps, while on battery power, input can be 12.6 - 11.0 volts. So amperage will go up slightly as the input voltage drops overnight.
Remember that the camper's refrigerator, CO meter and propane leak detectors get together and draw about 35 AH from the battery every 24 hours. So this load will not be significant compared to that load.
I have a pair of 120 watt solar panels, and each can put back about 35 AH daily.
SunELec.com Look for a 12 volt nominal panel with a aluminum frame. They had some 140 watt ones for only $229 last year. The price may have gone down since then.
Good luck,
Fred.
