Using 12V while hooked up to the tow vehicle

OldBiscuit is correct in my mind. None of the other suggested culprits would produce the same conditions as reported. In short, this is just a dead battery.

Watch Harbor Freight's ad and get a meter free.

I have three or four of them and if one gets wet or messed up I throw it away. (When things get critical I also have a $85 Fluke meter.)

Check out the most likely culprit - Take your battery to one of the auto parts stores that will test it for free. My money is on a bad battery.

If it needs to be replaced, measure your battery box, then visit Walmart and get the largest 12v DC battery that will fit in the box. Hopefully that will be a Grp 27DC.

I would turn your TV around and use jumper cables to the rigs battery.

For what it's worth info may have nothing to do with your situation - there is normaly a ciruit breaker of fuse breaker on the battery main line feed and and if it goes all the feed from the TT battery is lost. Some can be reset others need to be replaced. Only about $10, available at any Auto Parts Store. Normally located on frame near the battery case. If this thing trips or blows and your not on shore power (not sure about what would work if connected to the truck) you will have no 12 V power in TT. Easy to change.

sofraj wrote:


A couple things: as far as I know the battery never discharged very much. I've always removed it and placed it on a charger which is also a trickle charger.

I hope that charger really is a smart battery tender.....as a cheap trickle charger can do more harm over long periods than it does good.

Then.....the battery or the charger (or both) could have gone bad while it was out of the RV.

If you are going to continue to chase this problem on your own, as well as future power problems and questions, YOU MUST HAVE A METER.
They can be had for about $15.

Thank you all so much for your comments and advice. I'll have to go through this slowly to take it all in. I'll let you know how it works out.

A couple things: as far as I know the battery never discharged very much. I've always removed it and placed it on a charger which is also a trickle charger.

I don't think this model has a battery cutoff switch.

I replaced all the fuses whether or not they needed it (they didn't).

I was thinking of taking out my car battery and trying it. That should work, shouldn't it?

Anyway, on to my studies...

Blown reverse polarity fuses on converter......NOTHING will be able to power up the DC Dist Panel with fuses and individual DC circuits.
NOTHING....not the battery, not the converter and not the 'charge' circuit via umbilical cord

Main in-line fuse or DC circuit breaker on POS cable blown......again NOTHING will be able to power up the DC Dist Panel


OPs trailer battery was NOT fully charged using trickle charger
OPs trailer battery not being fully charged was unable to power up DC Dist Panel
OPs trailer battery being supplied a charge voltage via TV thru umbilical cord was able to supply DC Dist Panel......

Charge trailer battery and IT will supply DC Dist Panel w/o need for supplemental power from TV thru umbilical cord
Provided trailer battery charges fully and HOLDS a charge


(TV Charge line via umbilical cord can NOT support the load from DC Dist Panel.......wire from TV too small, too long and limited to maintaining a charge on trailer battery)

I will continue to remind RVers that pin 4 of connector does a poor job of charging your house battery. Better than nothing, but still poor. Your house battery is located a long ways away from you vehicle battery and the wire they typically use is much too small.

If you are going to be boondocking (no shore power) and you are relying on your vehicle to fully charge your house battery while driving, get a DC-DC battery charge. Not very common but the do exist ! They will provide adequate charging voltage even if the voltage on pin 4 from the towing vehicle is as low as 10V.

I like a test light over a multi meter, easier to use

There is a battery cut off switch somewhere.
and a Fuse or circuit breaker

Suspect list is as follows
Dead battery (Test light connected direct to battery will be DIM or Dark
Bad connection. Test procedure follows
Disconnect... Disconnected... (Switch off)
Tripped manual breaker/blown fuse, defective auto-reset breaker.

Diagnostic procedure description is using a test light, but if you use a multi-meter BRIGHT = battery voltage (12.6) Dark (Dark or dim) = less than 10 volts.

Check battery (Clip to one post, tip to the other (if six volt pairs check the most positive and most negativfe, ignore jumper unless light is dim/dark)

Move clip Chassis ground tip to POSITIVE

Move out along the POSITIVE wire, at each point if you have to move the clip check last known GOOD test point.. NOTE you may use an "extension" wire so you don't need to keep moving clip.

when you come to a device, check both sides of it, If it is a Solenoid (they look a bit like ford starter solenoids but are different, check only the LARGE terminals, ignore the small ones. Keep going till you hit the fuse block and check the main fuses there.

problem exists between BRIGHT and DARK