Did you mean to say that your motorhome was built on the F550 chassis? According to the literature I found online, that's the chassis that Newmar built the 2016 Baystar Sport 2903 on.
If that's the case, you've likely missing the fuse that powers up the 12-v feed to the 7-way that many use to keep a trailer battery maintained. According to the owner's manual, the fuse should either be a 40A in location # 10 or a 25A in location # 90. These can be found in the power distribution box in... view full answer...