Hello Thermoguy.
Without understanding the laws in your state, could you simply pull your horse trailer behind your fifth wheel? I suppose it would be tricky due to the weight of the horses/trailer and getting the fifth wheel set-up properly, in addition to the overall length.
Something you may want to consider as well is once you get to where you are going with the new motorhome and the horse trailer, how will you get around? My parents recently entertained the idea of moving from a fifth wheel to a motorhome to solve a similar dilemma with their boat. The two reasons they did not go forward with that plan were getting around when they got to their location and the cost of maintenance on the motorhome.
As far as recommendations on your class A search, it seems like you are already considering some of the key factors, like making sure the powertrain is up to the job. I'd recommend, as another did, to get into the large engines (11 liters plus) which are also used in over the road trucks. The downside is, for example, an oil change will run you $500 yourself (plus the $300 drain pan you will need for that job), or $1200 at the dealership. The annual costs continue from there add quickly up to a few thousand dollars on a "cheap" maintenance cycle.
Additionally, I'd recommend to look for motorhomes where "fixes" to common issues are "built-in". What I mean is look for coaches that don't have historical problems or poor designs in certain areas, but instead were designed and built right the first time around. This narrows your eligible makes to just three to five in my opinion, and may push you into older coaches based on your price point. However, I wouldn't worry about that because you will find many well cared for coaches with all of the options you want, and probably more, plus the pride of having an awesome coach.
A side question, how do you ensure your horses, regardless of what is towing their trailer, aren't being suffocated by noxious fumes from the tow vehicles exhaust? Would that potentially be worse being behind a motor consuming much more fuel and air than your current setup? I had never considered if that was an issue or not...
Good luck!
