In order for the coach to move, you will need to replace the front yoke of the driveshaft with a slip-yoke type, unless the parcel van had a transmission mounted parking brake. If this is the case, you can use the manual lever that came with your Airstream on the back of the transmission. You will just need to make sure you apply the parking brake every time you park, else it will leave without you. I don't think it's a real good, or even particularly safe, solution, but will get you "on the road" for now.
Your second question, the 6.2 is not a powerhouse, but it does have the torque to get over any hill I can imagine in Great Britain, albeit not as fast as you might like.
If you think it will be a major problem, you can get a GearVendors underdrive unit for your coach, it will lower the final drive ratio by 20% for starts and going up hills, and allow upshifting to the regular gear ratio when driving.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.