I like BFL13's answer best.
I like the solar on the barn idea a lot. I have installed similar on the winter storage shed for some boats. It worked well. It will not take a lot of solar to do a good job as you will have lots of time at low load to charge the house bank.
If your coach currently has a dual diode isolator, then leave it alone. Systems that have that also have a voltage sense on the engine side of the isolator so the diode drop (0.6V) is no longer an issue. If it has a single diode isolator (only two posts - very rare) then by all means, replace it with a contactor.
Depending on the main engine alternator, you may or may not be capable of doing significant recharge on the road. Alternators can be upgraded, but if you have to go past the largest parts store unit, this gets expensive in a hurry.
An 8D in AGM will use a lot of your budget and be very difficult to more (~170#). Yes, golf cart batteries will have more junctions to maintain, but at least you can more one without a crane.
Plan to install a Trimetric or Xantrex battery monitor. This will tell you what you need to know when you need to know it. Unfortunately, this will burn half your budget in one shot. Maybe plan it for next year.
We did this sort of work for the owners of expensive boats for years before the depression ended the business and all the big boats that could left the area.
