I had a similar problem with my Winnie....sat for almost two years on grass, and the brakes rusted to the point that when I took it down for an annual state inspection, I couldn't go over 45 mph!!
It cost me over $2K for the brakes to be completely replaced. I looked at the parts, and maybe they could have been turned, but the time and effort pretty much offset the cost of replacing with new. I also have less than 30K miles on the MH.
Now, I take the RV out every month, run the genset, and drive about 5-10 miles, if we don't take it out camping. That was an expensive lesson, that I don't want to repeat!!
If I was you, I would probably have the front brakes repaired.....then take the MH out and test drive it. You REALLY want all your brakes working properly, so test to see that it has stopping power. You can always go back and have the rear brakes done at a later date.
Best to find a Mechanic/Shop that you trust, or that friends trust. It's easy to get ripped off.:(