I had the exact same, called Magnum and they had me remove and disconnect the remote panel and plug it in the inverter directly with a different phone cable, no go. They said it was the remote. So I bought a remote, put it in, no go. Called Magnum again and they said it must be the control board in the inverter. Would be nice if you could borrow a remote from a friend to determine which one it is.
I ended up replacing the control board myself, about 2 hours. You must disconnect the shore power, disconnect the batteries and disconnect the phone cables from the inverter before you start removing panels on inverter. The instructions that came with control board are good...
Looking for small Class C, sold Newmar MADP