This may sound a bit far fetched, but have you ever considered a self funding project? Try Googling "self funding sites" to get a handle on what it takes to do this. Basically you put your idea out there at a specific website and honest to goodness people donate to your endeavor. Maybe if someone in your group has the talent to write lyrics they might be able to put together an acceptable outline for the fund raising project.
The reason I'm suggesting this funding idea is because I seriously don't think you and your band mates are going to be able to obtain a decent RV for $4,000. Sure you might find some old 20+ year old RV to ride around in, but it's not going to be fun unless one or more of you are handy at repairing various things that can go wrong on the RV. Plus you need money to even get parts to help repair it.
If your band really has the talent to play at enough venues to make a basic living then you may be able to find some people willing to help your group make it become a reality. You might consider a target of raising maybe $10,000 or more. That amount along with your $4K may make your dream come true. I've never undertaken a self funding project myself, but I've read about them and you never know unless you try.
I seriously with you good luck with your band and your hopes of touring together.
