This is (tranny mounted parking brake) a throw back to older truck systems. It is really not a big deal to repair any portion of the parking brake system. If the transmission is working properly (when full of fluid) you CERTAINLY don't need a new transmission.
To get to everything, loosen the two intermediate driveshaft hanger bolts (first shaft hanger AFTER the transmission) and than remove the four driveshaft flange bolts to transmission output companion flange http://static.zoovy.com/img/racerwal...fff/m4851a.jpg
and desengage drive shaft (might take a small pry bar to release it from the flange). Than finish removing the intermediate hanger bolts and lower drive shaft to ground. (may want to have a freind under the coach when you do this)
Release the parking brake handle in the coach and tap (gently) the parking brake drum off of the output flange.
Remove the linkage to the brake pawl actuator (clevis pin) and remove spring hardware and brake shoes. (shoes may be sent to local brake reliner for pad relining) Now for the hard part. You will need a PROPER sized socket (there is no standard size) with PROPER circumference relief ( a thin wall design will work if you can fine one) to remove the companion flange from the transmission output shaft. It is splined to the shaft and may be somewhat cantankerous to get off.
With the companion flange off of the shaft you are ready to remove the mounting/backing plate for the parking brake (usually 4 bolts) and than the transmission rear tail shaft housing.
This is a representative layout of your mechanism.
and here is a drawing view http://www.fordification.com/images/...seomatic02.jpg
Once the tailshaft housing is removed you need to have a transmission shop install a new seal AND housing bushing.The seal without the bushing will do you no good and the leak will still be there soon after the repair is made.
Once you have all this done you need to take a look at the companion flange seal wear surface. That is the bright shiny machined surface in the above picture. If the surface has a groove worn into it the new seal will probably leak also. A new companion flange is not cheap but the seal surfaceMUST be dealt with before reinstallation. They have specialty repair sleevs to install over the old worn area of the seal surface that will restore the seal surfcae to new condition. Speedi-Sleeve shaft repair kits
They cost in the area of 10-12 buck wereas a companion flange will run you at least 85-100 bucks.
With all the new parts gathered it is time to replace everything in reverse order. Adjust the parking brake to drum clearance with the drive shaft remover so that you can "feel" the lining drag on the drum in order to keep from getting the brake too tight and causing then lining to heat up and chrystalize.