It all depends upon whether this girl is the one. Is she the one who makes your heart pound and your head spin, the one you can't do without? If she is, you need to do everything you can to make her happy. Don't try to force her to live life your way.
On the other hand, a girlfriend is not a wife. If she is just someone you have fun with, but not the one you intend to spend your life with, ask yourself why you're with her anyway. How much do you have in common? Do you have other shared interests? If she is just a pretty face, but you have no shared interests and she isn't the one who moves your soul, why try to force this?
Most guys sort of brush around the edges of questions like this. I used to do counseling, so I am going to play Dr. Phil for a minute. I have been married almost 30 years and am even happier today than the day we wed. I am going to ask you to think about some serious questions. I am not saying you are this way, but in that you broached the question, I think these questions for you are worth asking. Do you have a need to decide what the two of you are going to do together? In other words, do you have a need to control the relationship? How much say does she get to have? If you have a deep need to control, and she doesn't want to be controlled, either you have some growing to do, or otherwise you need to look for a very submissive woman. You can't make someone become what she doesn't want to be. If this is that big of an issue for you, you need to do one of two things: 1) let her be who she is and love her as such, or 2) look for someone else. It behooves you to look inside your own self and figure out the answers to these questions; don't try to psychoanalyze what's wrong with her. In the long run, what is more important to you-a fulfilling and happy marriage, or a female to ride around with you in your Sportsmobile?
Now, let's say you both love each other and you simply don't see this the same way. People in religion and politics these days live by the creed, "No Compromise." That is a recipe for disaster in marriage. Look for ways you could compromise with her. Maybe the SMB is too small for her. Could you live with a Sprinter SMB that doesn't have 4x4? Could you go with a Provan Tiger, an EarthRoamer, Host Super-C, Xplorer Xcursion, Krystal 30 or 33, Suncamper Sherwood, AC Landcruiser 80, or an out of production Toyota Chinook, Serro Scotty, Revcon Trailblazer, all built on a 4x4 pickup? They can't rock crawl like a SMB, but some
of those can take unpaved and rough roads pretty well. Then there are even bigger expedition vehicles built on full sized trucks. Provan currently has two pickup based models with a third in the plans. EarthRoamer also currently has two pickup based models with two more in design. The lower end models are bigger than an SMB, but comparable in price. Some of these have a wet bath, and some have a dry bath. Can you expand the variety of vehicles you are willing to consider, in order to make her comfortable and happy? To learn about the full range of expedition vehicles, have a look at: http://www.expeditionportal.com/forum/