I want to know what to check on my 1988 F150 truck that might be running down the battery. Everything that I know of is off including interior lights but I'm still drawing amps thru my volt meter set at DC 15 volts. Am I doing something wrong?

 

There are several circuits that are Hot at all times; try to isolate by disconnecting accesory wires at battery term of solenoid or by pulling fuses for various circuits one at a time. some likely culprits are lighter feed (bad lighter element contacting terminal in housing socket), Paticularly check if you have cruise control dont know if 88 had the brake pressure switch that has been failing. The computer has keep alive voltage to keep the memory parameters. If you have water leaks into cab anyplace where lighting switches could get corroded (overhead dome lights, door jamb switches, etc) power is available to many of these with ignition off.
As is stated in many electrical posts verify your grounds are good look for evidence of corrosion and be sure the positive cable is is not internally corroding - it can do that without significant external evidence but will be swollen and fulll of green moungue that will put you on the side of the road.
Look inside all fuse boxes for evidence of water or corrosion also headlight switch and dash controls. Post back any thing you find and someone will try to help. Good luck.
Also if you are on a 15 v scale on a meter you can not read current draw - you have to switch to a current scale if available and disconnect the circuit and put meter in series in the circuit unless you have clamp on probe capability for you meter. Most multimeter current scale are low current capabilities, some may go as high as 10 amps but require using a different meter connection to use that scale - read the instruction manual for the meter you are using to verify the application.

 

Everything 95X said. How much current are we talking about here?
If you are reading volts from terminal to disconnected cable that would be normal.