Very simple to diagnose but most mechanics just do not understand the electrical systems.
You really need an ammeter inserted into either lead of the car battery with the lead disconnected. A clamp on meter will not be sufficient unless there is a heavy flow. You may be looking for a 1/2 amp or less draw.
Answer downtheavenue's questions first though.
Working on vehicle electrical systems for 40 years I never say never to strange problems. All things are possible. I would like to know if you have a braking system on the car and if so what kind.
IF any of the folks looking at the problem used a statement such as (it has a 12 volt draw), move on. They have no clue what they are looking for. IF they cannot tell you how much current is being drawn they do not have a clue.
IF you want to chase it yourself, watch several you tube videos on how to measure current and then buy a meter.
With the vehicle unattached disconnect the negative or positive cable of the battery and measure the current. The connect the pigtail and measure again. If you see any increase in current you have an issue. If not, connect the battery again and start the mh. Measure the Voltage across the battery. You should see 13.5 or so. If 12.7 or less then the rv is not charging the battery. Start the car and you should see the same higher voltage.
