Help please - first use of panel with rv results in smoke from inverter

That does sound like the polarity was reversed. If you don't have a multimeter, I suggest getting one so that you can check the cable polarity next time before making the connection, and so that you can measure things like the voltage on your battery to see if it's charging. You can also test the panel open-circuit voltage and short-circuit current (if the meter can read up to 10A, your panel will probably give you around 5A).

Is the inverter connected directly to the battery? When did you see smoke? Did you possibly remove and reconnect the inverter with reversed polarity?

The smoke appeared after I put on the extension cord. The Rv's onboard inverter/charger is/was connected correctly, but was not plugged in at the time. I have to check if for some reason the cord reverses polarity.

so the extension cord does reverse polarity, is it just this one or do they all do that? I would suggest being careful using one to all out there. The battery (which was unfortunately almost completely discharged (that's a a whole 'nother genius story), now reads 92% when hooked up to panel, been charging in good sun for two days. Panel says "3.1v"... is that a normal reading in full sun for a 100 watt panel? The little usb things don't work to charge my phone... that's where the smoke came out of. Just wondering if this controller is still ok.

Panel in full sun should be about 18.0V

Don't feel bad - I got burned by that exact same scenario when I was first starting out.

Unfortunately, there is no standard with SAE connectors, so manufacturers are free to wire them up just so they work with their own gear. What's infuriating is that two or three manufacturers will wire the same way, yet a third one wont. OR, I have seen them "fix" the polarity problem by purposely wiring the panel in the wrong polarity in the first place as a total kludge.

SAE connectors in solar are also a total no-no. They are frequently high-resitance due to poor contacts, and/or high-resistance from corrosion over time.

I recommend using Anderson power-pole connectors for these little panel jobs. If you don't want to make the cables and connectors yourself, they also sell them in pre-built cable configurations.

Unfortunately, you may have smoked some diodes in the panel. In your folding panel, you may see two or more junction boxes on the backs of the panel. Open them up and inspect for burnt / melted wiring, and more likely, blown bypass diodes. Replace them once you identify the diode make and spec. You may need help from the manufacturer in this.

And yes, invest in a good multimeter so you can spot polarity mistakes, meaure voltages, check diodes and the like. You may want to chalk this up to a red-flag if the folding panel manufacturer was using sae-connector wiring - which typifies ultra-cheap products.

Sending the panel back for (non-warranty) repair would be expensive (labor, parts, shipping x2). The OP will most likely need help from someone who knows their way around electronics. If he does not own a meter he most likely will not know how to replace diodes. Best bet is to find someone who can diagnose/fix the problem locally. The cost might be more than the folding panel is worth. The parts are cheap, labor is not.