If you have a regular vet who already knows your dog, give him (or her) a call. Explain that you didn't want to traumatize her by driving her in, and ask him to call the mobile vet for more information. That way, the results from the exam and testing will be in her regular chart, and hopefully your own vet can explain things more clearly. Better yet, go ahead and take her in anyway. It doesn't sound like she'd be too traumatized with a car ride at this point. I wouldn't take the mobile vet's answer about surgery as the final opinion either - that advice would be much more reliable coming from the vet who already knows her.
If your dog was never spayed, it's possible she has cancer, but there are plenty of other things that could cause this kind of bleeding. A bad infection of the uterus can do that, but it requires surgery to remove the uterus. Don't give up now. Get her to another vet FAST before she really starts suffering.