Did you troubleshoot by testing with a different trailer? Maybe try a simple small trailer to confirm there's not an unusual draw on the trailer side. It may not be a problem with the van... You can probably just pull up to the small trailers at tractor supply, etc. and plug in the harness to test - or spend $25 at harbor freight and buy LED trailer lights to plug in to confirm the issue.
It sounds like you may be grasping at straws and here's a straw - use all LED lights on that circuit (all running lights on van & trailer) if you suspect a large load. Those will draw less power than incandescents, last longer and illuminate faster. Any alternator from the last 70 years should power a few additional running lights so don't worry about not having the high output alternator. There are a couple threads on installing LEDs if you'd like to try...hopefully your new trailer is already outfitted with LEDs being new...
It may be helpful to read through the wiring diagram shop manual ($95 to buy) or subscribe for a period of time to understand more of what's going on with that circuit - there's a shop manual thread with the details.
PSA for others - if you know you'll be doing towing, do yourself a favor and get the van with the tow package. There's enough capability and convenience included to make it worth your while (pre-wired for brake controller, sway control braking, beefy hitch, electrical is all done, larger radiator)
I'm a big DIYer, so I applaud the effort to customize one's vehicle, especially for practical reasons - best of luck
2018 TL+ Jazz Blue / "S" Appearance / Advanced Safety Tech / 8-seat / Towing / 18" stock winter wheels / 20 Speaker HK / spare tire / splash guards / keysense. Ordered 4/6/18, built 4/13/18, Upfit complete 4/25/18, Arrival ETA available on 4/30 (5/5-5/15), unloaded from train in Jessup, MD 5/6, trucked to dealer 5/16
Mods: 19" MSW Type 48 Black wheels w/wider 255 tires, LED conversion, Husky Xact Floormats, Ceramic Tint, aukey DS02 dashcam
Next up: Shop manual
Last edited by twig2121; 07-06-2019 at 08:13 AM.