I run a hard wired Power Watchdog
. I also carry a replacement surge module. It is kind of nice, it checks out the power and if it's good it connects. Really like having smart breaker,
I agree totally invest in a better extension cord. The long run with small wires will reduce the incoming voltage. So if your just keeping the Van hooked up to float the battery, you really are not pulling a lot of current. Depending on how it is configured, the charger could be at 25 amps or 100 amps. That should be set based on the size and kind of battery. Even if set at 100 the 20 amp circuit would be fine.
There are also some limiting settings on your inverter/charger, that may limit the output of the charger if you were pulling heavier loads. This wouldn't help you with your 20 amp input.
To understand the affect of the small sized extension cord look at these .
Charger set at 100 amps
Absorption Voltage set at 14.4 volt
10% conversion cost
(100 X 14.4) *10% = 1584 watts
1584 watts / 120 (incoming AC best case) = 13.2 amps
1584watts /110 ( AC input challenged by long small extension cord) 14.4 amps.
And that would be a best case example, because the conversion cost would get worse at lower voltage also