X2 to what B and B
If your battery has been sitting idle, for 1.5 years, it could likely be "sulfated" or otherwise not able to take a charge / come up to full voltage. There are other possible explanations but same symptom / problem.
The "tech" of it is, there isn't enough current, figuratively enough water pressure coming out of the pipe if you will, to spin those motors under their full load. The motors will try harder but with the low voltage, they will draw more current, which will heat the wire in them more than design spec, and can cause failure (expensive repairs.)
The fact you ate (blew) a fuse (circuit breaker) supports the low voltage theory. This is a protection feature operating as designed.
The service manuals will indicate that the mechanism "may" operate under the lower voltages but also say that there will be increased current and problems as just noted above. In some cases (i.e. trying to use "jumper cables" or just the converter,) there will not be sufficient current.
The 12 volt connection from the truck to the camper has similar issues, voltage drop (length of relatively thin wire) and low current availability (relatively thin wire.) Think of how thick the primary wire is from truck battery to chassis and starter.
You could temporarily swap battery from TV to the Camper, BEING CAREFUL ABOUT POLARITY + / - connections of course, and that could provide enough electricity to get the sliders back in. Again figuratively, this is like using a fatter, shorter hose instead of a long, skinny garden hose, you get more water.
There are also manual methods for getting the slides and awning retracted; our chassis manual does not have info the levelers.
Actually found "youtube" videos of some slide and awning procedures.
Best Luck ...