Well.... This is an interesting thread. Let me first say, I know zero about TT's as the name says. I have purchased a new TT about 4 weeks ago so my knowledge is zero, although learning. Going back, many pages the op's post was 3 things( that I recall).
Marker lights(trailer lights) zero insight on that, zero electrical knowledge. My father in law installed hitches/trailer lighting etc for 20 or so years after the navy and can fix those in a heart beat. Past experience with my boat trailer usually left me with a bad/loose ground wire.
City water leak. First time out in my new trailer, I hooked up to city, blasted it open. Went into trailer, went to get sweater out of closet next to bed, foot got wet(leak). What I found out is that the dealer told me I needed a pressure regulator on city water, which I had bought. Screwed it on without doing anything else problem solved.
Black water flush leak. Same problem as above, used pressure regulator, leak gone.
After reading winterizing posts about controlling psi/etc I only assume that in an effort to reduce weights/etc that the mfg's are using lighter weight hoses and less rigid fittings leading to minimize pressures to so that fittings and hoses are not compromised. So far, 1 trip, I have found 2 cosmetic issues(could be from transit or from being on showroom floor and many walking thru) and no true mechanical or workmanship errors. The true test will come when the 4 of us spend the week if thanksgiving in it at the smokey mtns.
2012 Lacrosse 303RK
2012 F250 6.7 diesel