F350 battery drain

I've got a F350 chassis on an RV. The chassis battery keeps draining and than won't take a charge ( the battery goes bad) Have replace 3 in 3 years, last 1 lasted 2 months.

I did find a drain with a test light between negitive post and cable. the test light does not stay on it fades out in 1-2 secounds. It is a 40 amp fuse for (courtesy light relay contacts,cluster memory,BB battery feed) i don't know what these things really are, or if this is normal. I'm I barking up the wrong tree!

Thanks

I definately would check out the battery drain. The door light switch is a good possibility. Unplug one item at a time on the fuse until you find the problem.
If you don't drive the camper frequently I would buy a battery maintainer and plug it in while not using. I have a diesel tractor and I don't use it much in the winter. I installed a battery maintainer I got at Auto Zone for around $25.00.
I still have to clean connections during spring maintance. When I lived in Ohio my tractor battery had to be replaced about annually even though I charged it a few times in the winter.