I am sure your motorhome has a generator. Since the Journey is a DP, I am 99% sure it is a diesel generator of at least 7.5KW, so it can run your A/C and everything else. If so, it will use about 1/2 gallon of diesel per hour or about $2 an hour. How much do you plan on using it? The generator should be run every once in a while, under load for 2 hours, to keep it working right. Monthly is best for exercising your generator, but with a diesel you can stretch it to 6 months. Why I ask is since you have a generator, you have flexibility and can start with less solar, which means less money.

Edit: We have friends with a similar motorhome and 1200 watts of solar.  They have an RV fridge, so they have 60 to 100 amp-hours more battery margin than you.  They can go without running their generator indefinitely.  But when it gets hot, they run the generator and air conditioning.