If your converter is going dead with no battery in place and only a few lights, like 3 to 5, then this really seems to point to the converter.
If you have the standard 921 light bulbs, looks like this
921 - 18 Watt, 12.8 Volt, 1.4 Amp Miniature T-5 Bulb | Bulbs.com they are 1.4 amps each. 5 lights on at once is 7 amps. That converter should have no issues with 5 lights on.
If you want to try this to confirm, put the volt meter on 12 volt terminals about anywhere in the camper. Even the battery leads that are unhooked. Tell us what it reads? Is it 13.25 or 13.65?
And then 1 at a time, start turning on light bulbs and watch the voltage, 1 at a time. Tell us when the voltage starts to drop and how many bulbs are on.
One thing for sure, on an incandescent light bulb if it is start going dim, the voltage is going down. This should only happen when you start going over 35 amps. With nothing on in the camper except the LP detector that would take close to 25 light bulbs of the 921 vintage. I doubt you have that many in the entire camper.
Hope this helps
John
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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
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