What is this???

It is a 20 amp manual re-set circuit breaker...

I prefer the self re-setting circuit breakers as they will re-set after a few minutes.

Here is a link to instructions on how to post a photo.

Mine looks like the pic above, except no reset button. They are auto resetting.

Assuming it is similar to above. If there is a wire direct from the battery to the circuit breaker, then it should be hot on both posts. If hot on battery side but not the other, then bad breaker or needs to be reset if manual reset. If not hot on the battery side, then a bad connection between the breaker and battery.

There are three varieties (and of course many current ratings) of these circuit breakers, besides a few different styles of mounting. The most common are self-resetting after a shortish time delay (I assume basically once an internal bimetallic strip cools down and makes contact again, not unlike a christmas tree light flasher bulb).

There are some that are manual reset, as in the picture that Chris posted, with a little button on one end that pops out when tripped.

Finally, there are some that will self-reset, but only when power is removed. I don't think these are too common on motorhomes, though perhaps they should be.

I wouldn't be surprised if the slides/jacks have a manual reset unit; my steps were (originally) wired with a manual reset breaker, though I've changed that to a plain old fuse for reasons that are not at all pertinent here.

Wow! you guys are geniuses. But, looks like it was tripped because the main slide is dragging. I need to go check that out. Thanks a bunch.

If it looks like this:

The small button on the end is the reset.

Auto resetting circuit-breakers.

This look like what you have Bussman auto-reset breaker