House batteries are charged but wont work.

Just got back from the garage we took the RV to and its another one for the book of weird garage experiences. I have used this garage/mechanic before and they have done good work in the past but this was weird even for me. So after being told that all was finished and everything was good however the generator still isn't firing up and it shorts out everything when you try we at least were one step closer with the house batteries to work only to find out the mechanic who worked on it just hooked up a direct line from our engine battery with a fuse to the solenoid. First they charged us around $250 for parts and labor then took us over what they had done. What a complete waste of time. We explained to them before and when we showed up what our problem was and what we needed done. Anyway we are left with the stress of being jerked around and still no house batteries. All they could tell us is that the generator was potentially the culprit. Something to do with it shorting the power. But I can say they clearly were in over their heads with this one as am I. I plan on going over all the wiring from the generator to the house batteries to the solenoid to the main switch see if I can figure it out otherwise I am afraid I will have to put my faith in new mechanic that can hopefully help me out.

Open to any troubleshooting walk throughs that would be typical for this type of thing. Would like to know at least a good place to start. I will look more into this momentary switch and relays and suggestions on how to go about checking them or what I'm looking for.