I am afraid that I am not going to be the bearer of good news on a lot of the questions you have asked. Since I cannot see your Motorhome I cannot answer a lot of your questions or even give you a guesstimate of how much it will cost to repair it, or even if it is feasible for you to attempt the repairs yourself.
Before you do anything else you need to have the damage to your RV inspected by a Certified RV Technician. The fact that you see water damage to the walls and ceiling in the interior of your Motorhome means that it may have been exposed to moisture for a long period of time. This could indicate that the structural integrity of your RV has been compromised.
Since I do not know the extent of the damage to the roof of the RV there is no way for me to tell you whether you can safely walk on the roof without causing further damage to the roof of the Motorhome.
Depending on how long the Motorhome has been leaking there could also be mold or mildew inside the walls and the ceiling.
The only way to find out for sure what needs to be done is getting the inspection done. Once you are told what has to be done to repair the Motorhome you can then make an educated decision on whether you can tackle the repair job yourself.
Sorry that I could not be of more help.
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