12 Volt system problem

Thanks for the help and suggestions everyone. @Bordercollie, My skill level with RVs is higher than the guys that work at my local RV dealer/service center. Everytime I go there I'm offered a job. Example, The last time I went there I took a circuit board from my furnace in my TT to them to use a circuit board tester to see if the board is bad. They tell me "We don't have a tester, if we think the board is bad we just plug in a new one". This kind of practice is why so many people test everything at home and it works fine then get where they're going and the furnace or whatever fails again. Then they take it back to who worked on it and they say the board is bad again must have been defective and install another new one. Which of course will work fine until you get away from home and start using it.

This is my first motorhome but I've lived in and worked on TTs and TCs for years. I've been living in them full time since 1992.