It seems odd to me that with a shorter motorhome you have such a rear weight bias. Where are your water tanks? If they are behind the axle, make sure to travel with them empty and fill up when you are at location if possible.
You could contact a local shop that makes awnings and ask about replacing your fabric only.
I couldn't agree more with twins89. Roof leaks are a HUGE deal. Eternabond is the premier (read expensive) fix for long seams, but Dicor self leveling lap sealant works well and is affordable. Dicor is definitely the standard for sealing up things like vents or "curvy" seams. Dicor on long seams is fine, it just won't be as pretty as the Eternabond, nor will it be as easy to inspect for leakage.
If you want to make the rear axle "stronger" you can add airbag assist springs for a couple hundred dollars.
When you hookup to water, figure out where your input line is and see if you can get eyes on that fitting from inside the coach. Then check every faucet and toilet to ensure the gasket at the threaded inputs don't leak. Gaskets are cheap and easy to replace. Last thing to check is your main water tank and the water heater fittings. Make ABSOLUTELY sure everything is leak free under both city and pump pressure. Water damage is THE killer of older motorhomes. A nice thing to have is a pressure regulator that threads inlie to the end of your city hookup hose just to make sure things don't somehow get overpressurized. Mine came with my rig, but they are pretty cheap online.
Best of luck and have enjoy your rig. :-)
Jose