Hi SWK: I know how disheartening it can be to realize that all that money just goes to gas. It's halted us from taking a much-wanted cross country trip. There's a lot of things involved in your trip. First of all, you got the RV, so you sure want to use that instead of flying. And, what a scenic trip it would be! Plus, you want to bring your dog with you. You can't put a price on reconnecting with your sister. It is something that you really want to do.
Aren't there other ways that you can cut corners on costs? Perhaps boondocking at Walmarts, rest areas and truck stops instead of staying at RV parks. Or, at least, alternating nights. Drive 55, the slower you drive, the more fuel you save. Don't pack too much, and don't drive with full holding tanks. Also, eat more peanut butter and jelly sandwiches and less going out to eat along the trip.:)
What is your financial situation? If the trip is going to cost $1,500-2,000 more than you anticipated, is there any way that you could just use the next 12 months to pay 200 extra dollars to your credit card? If you are starting with little to no credit card debt, then it's feasible that you could do this excursion and not suffer any big financial repercussions from it.
Here is a question you might want to ask yourself. This is what I pondered when I have been in similar situations. Will that money matter five years from now, or will the trip? I'll give you an example. My hubby and I took off on an 18 month RV trip back in 2002. Less than 2 months into the trip, our engine seized up and broke down in Connecticut. We had to decide whether to continue on, or let the cost of the new engine stop us. We decided that the engine wasn't going to stop us and we slapped it on a credit card and continued on our adventures. We never regretted that decision. Five years later, that money meant nothing to us, but our memories did.
Write out the pros and cons of each of the 3 decisions you made up there. It sounds like you don't want to travel any other way but by RV, and I totally understand. It also sounds like you would be really sad to not take the trip. And I get that too. Be creative with other ways that you could cut costs, and also see if there is any other way that you can figure out how to make that extra $1,500 not sting so much.
Hopefully one day in the near future, all of us RVers don't have to deal with the whole gas prices taking up most of our trip budgets.
Keep me in the loop!

Erin