As you suspect, a real 16.5 volts left on long enough will start cooking the battery.
I have not been into one of these, but I found these two things.
This U tube while long and not totally accurate, he did show a pot on the charging board. Not sure if that is a charge voltage tweakable pot or not.
This link turned up a manual https://d163axztg8am2h.cloudfront.ne...549dd4fa19.pdf
That battery charger is not the greatest, any thoughts of an upgrade?
Let us know what you come up with.
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
Google Custom Search For Sunline Owners Club