IF all the 12 volt stuff works when you are running the generator or plugged into mains
But none of it works, and I mean none, not the inverter, not the furnance, not the isle lights not the light in the bathroom
none of it works when you don't have 120 vac present. Then the list reads like this:
1; Battery disconnect switch (See note 1)
2: Fuse
3: Batteries are show
4: Wireing is shot
Note 1; There are basically 4 types of battery disconnect switches.
My coach has a remotely operated latching relay, there is a control palel (in my case next to the steps actually below the floor but in the stair well, in other cases it can be most any place you have switches) Another simpler system I will be using for the inverter circuit has a knife switch usually near the battery, still another system has a "Quick Disconnect" on the positive battery post. And there is a rotary (knob) switch as well. Many options here.
Here is what you should do,, First find the HOUSE batteries, (usually it's plural) as opposed to the chassis battery (usually singular) usually these are not located in the same place. Find the POSITIVE lead, often marked with a PLUS sign on the battery, Follow the wire, with your eyes, looking for a disconnect switch or fuse, if fuse, use a volt meter or test light to check the fuse, by the way, use the test light to check the battery (more follows) all testing is done with no 120vac by the way as the converter will mess up the tests otherwise.
While you are there take a good close look at the NEGATIVE (-) side of the battery as well... Any time you are doing electrical work, and a good number of jobs that have nothing to do with electricity, the job procedure begins with "REMOVE BATTERY NEGATIVE LEAD"
Since this problem begin when you picked up from service... I'd almost, and I stress ALMOST, be willing to bet the technician dissconnected the negative battery lead............ and forgot to reconnect.
Back to testing... AC power off, connect test light across batteries, it should light, Move NEGATIVE lead to chassis ground point (any metal chassis part, IE motor home frame) shoudl still be lit, if not check negative lead, if lit move to fuse, or switch Should remain lit both sides of fuse or switch, if not check fuse or seitch.
If remote switch you will have to find the thing (See your manafacturer's manual for info) in my case the house batteries are under the entry steps, but the switch is above and to the driver's side of the stering column, in the engine compartmetn, Yours could be anywhere.
If the light is lit at every point near the battery you will need to find that switch, this can be a job. but the manafacturer has given us a clue... Sometiems. If you have a switch marked EMERGENCY START, and a partner, I can tell you how to use that switch to locate your remote switch,,, If not, then you have to use the hard way (Trace wires)
If you locate the remote switch (Assumign you have one) then again we do the test light thing, Step one is hook one lead of the test light to a good chassis ground, same as last time we did that.. Now test both sides of the switch (Actually there are 3 sides to this switch) all of them should light the light... I'll bet one does not.
If you get light, dark, light (or any combination of 2 lights, one dark) find the remote switch and operate it, you will hear teh relay click and 12 volt systems come alive (we hope) if not, apply mains power (AC power) you shoudl now see 3 lights, again try operating the remote switch, you may hear the relay click in.
NOTE: if the light is very dim when you test at the battery.. Either the batteries are shot or they are dead due to the remote switch being turnd off, not on.
But give us the results of the test light sequence and we may be able to give you more info