I believe your trailer has the same (or a similar) type remote control as mine. If it does, then your problem is a bad component on the remote control receiver. That is the "black box" installed in the trailer, probably in or near the battery compartment.
The "problem" is a safety interlock that prevents a "dual input" to the trailer components... As an example, imagine you're opening the main slide, using the remote transmitter and someone inside the trailer pushes the retract side of the main slide wall mounted switch. The motor would be receiving power both from the "OUT" side and from the "IN" side of the motor wiring. That would potentially damage the slide motor, so the manufacturer installed an interlock to prevent two signals on the same circuit.
Here are some photos of my remote control receiver. Notice the two sets of input wires along the bottom (where I'm pointing). If you disconnect one set of those wires and connect them to the same input as the other set, I'd suspect your slide switches AND the remote control will work. The problem comes in when (if) someone uses the wall switches while you are using the remote control. Essentially, what you'll be doing is "hot wiring" the wall switches to the same circuit controller as the remote transmitter is using.
Here is a link to the Lippert remote control systems data file: https://lci-support-doc.s3.amazonaws...cd-0003488.pdf
I believe your remote system is obsolete, but you can check with Lippert to see if there is a revised system that will work on your trailer. There is a part number on the remote receiver. Lippert Customer Service will need that number to see if your system is still available or if it's no longer stocked. Mine has been obsolete for about 4 years now and my trailer is a 2014, so I wouldn't imagine your system is available, but it would be a pleasant surprise if it were.
Here's the photos: