Well, it's intriguing to me because I know so little about it.

I made two changes to my camper on the same weekend. I installed two
(actually four, because each of the USB outlets has a 1 amp and a 2.5 amp
socket) USB outlets, front and rear, in my vanagon. They have a pilot led
in each, plus the rear one has a voltage indicator that has old-school LED
numbers.

I also bought a TDI jetta that weekend and quit driving the camper on a
daily basis.

I noticed that in two or three days I could not start the car and needed a
boost. I could see by the LED meter that it was dropping. So, I added
Harbor Freight's cheapest trickle charger (which won't charge at all, but
it will hold a charge in place up to about 13.1 volts).

After a recent overnight camping trip, I left an iPhone 6 charger cable
plugged into the 2.5 amp socket. When I went to retrieve it I felt that USB
end was HOT, almost too hot to touch comfortably. I removed the cable to
see if it was the outlet or the cable, and the outlet cooled down.

Is this normal? I think the 1 amp outlet is for phones, and the 2.5 amp
outlet is for tablets and such. I have often swapped those around
unthinkingly in the house with their respective chargers and have never
felt a cable get that hot.

I am trying to figure out what is going on here. I have no problem starting
after using lights and sink camping, and my batter is from November 2014
and has been taken care of, so while I can't rule out the battery it seems
OK.

I would like to know how much current one of these units draws, with the
with out the voltage indicator. I have a spare, I would mail it to anyone
who cares to do the test or I could be walked through it by someone who
knows what they are doing.

Jim