Epilogue -Sold our camper
We sold our camper last summer, almost a year ago now. We miss it.
The last service before putting it up for sale, I again spoke to the service advisor about "mushy brake pedal feel". He said they'd check into it. Part of the service included a service-interval brake flush.
Yes, the brake pedal felt much better. Not ideal, but much better. I queried the advisor and he offered almost indifferently "Yeah, they've figured out how to completely flush these systems".
He eluded to how Mercedes is now requiring that the master cylinder "system" needs to be flushed in stages. I couldn't get much more out of him. Not sure what that meant. We really liked the improved pedal feel.