Casita TT

Owner Manual
Page 179-181
http://www.fordservicecontent.com/Ford_Content/catalog/owner_guides/00f12og5e.pdf

F-150 if it is HD factory towing package the charge wire has been run to the 7-pin connector but the fuse might need to be installed.

If the light duty towing package, it would have had only lights connections to a four pin, and an aftermarket six or seven pin would have been installed for brakes. A line for 12V charge would have had to be run aftermarket, and installers don't always do that.

f150 2000 is the truck

Knowing what her tow vehicle is would help. GM is usually in fuse box under hood. Pos.1 I believe and is either a 30 or 40 amp square looking fuse. Sometimes break wire and or 12v connection is tucked in next to fender and has to be attached also.

she only uses the trailer occasionally. yes it is to have a charge battery when she stops not extended use.

Need to know where this fuse would be

I'm assuming you're referring to charging the trailer's battery off of the car's alternator. I don't know for certain (never owned a Casita) but the trailer almost certainly has the needed connection already.

Most vehicles with 7-pin connectors as part of their tow package do require that a fuse be installed in order to activate the 12V pin. Information about that should be in the vehicle owner's manual.

If the hitch and 7-pin connector were added later by a third party, that party may or may not have run a 12V wire back to it. They're not required, so it's anyone's guess. :)

BTW - if it's planned to do this regularly while camping, this is an extremely inefficient way to charge the battery. Cars usually don't output a lot of amps to the trailer connector. You'd have to leave the car idling for hours. (It's good to have for the drive out/back though.) For regular use while camping, solar or a generator are better choices.