I have followed your trials and tribulations over the last year. Fortunately you did not go to California by tow truck. I estimate you have put $60,000 into your motorhome in the last year. I would not give up now as I think your battery charging issues go back to the alternator and isolator replacement... Not done properly.
Alliance is a good choice but let them know what has been done electrically... New alternator and isolator. For 3-4 years it sounds like you did not have all of these electrical problems, at this point I think you're over the hump on spending money and solving this electrical problem should not be that expensive. Easy for me to say although I probably would've thrown in the towel before fixing the rust problems. Good luck and hang in there a little longer as now that you are retired it will be worth it.