Well Drt,
You got a set of answers from one of the tribe that gives good juice answers. What he was too kind to say is,"ARE You out of your Mind?"
A 3000VA inverter will require a forklift battery for support and real welding cable to attach it.....
Also as Mex said, the existing converter is probably junk. If it is a big heavy box, it most certainly is. Replace it first. If you replace the house bank with a pair of 6v golf cart batteries in series (to make 12v) and figure out what generator you can shoe horn in there, you will be worlds ahead. You should be looking for a 40~60 amp three or four stage converter/charger.
My coach is only 7 years older, and I can tell you about a lot of mistakes to not make. Idling the main engine to charge the house bank would be a good one to not make. First off, the alternator output isn't all that much, but then you also have to include the fact that nothing good happens when an engine is idling.
Now, about older coaches. They can be reliable. The only reason mine is not on the road to day is that the engine was getting weak and burning oil at about 95K miles. The thing to thing about is that all the rubber stuff is dying of old age. Not just the tires. I hope someone told you that tires live to a maximum of about 8 years under the best of treatment and that is with no regard for miles. I have see tires with mold nubbies still attached blow up...
All the belts and hoses should at least inspected and the cooling and brake systems both flushed and the fluids replaced. Also look at any rubber fuel line. Fuel lines have a double whammy. First is age that would be a gotcha in any case, but then the new "motorfuels" are real tough on old materials.
Shop locally for a good used generator if you plan to do anything but FHU "camping". To run a single A/C, you need about a 4kW unit. Just make sure it runs and the carburetor is clean before you shoehorn it into the space. There are lots of used units around because so many people have not taken care of the roof leaks (that is a hint).
If you have other questions or problems come on back here.
Matt
