Landing gear, slideouts are wired DIRECTLY to battery due to high amp draw (30A +)
Converter helps when connected to AC power by going into boost mode and sending high DC voltage to charge battery while operating landing gear/slideouts
BUT w/o AC power a good battery should/can provide necessary voltage to operate landing gear/slideouts
Your DAD's issue is bad/weak battery
And since you stated he is using YOURS then you might also want to check the converter 'reverse polarity' fuses and the battery positive cable to converter in-line fuse (or circuit breaker) to see of blown/tripped
Polarity fuses on converter
In-line fuse/CB should be in basement compartment (probably front compartment) close to battery ---just follow pos cable.
If either blown....converter can NOT charge battery