Repairing Motorhome seals - help please

Can anyone out there give me some advice on how easy this is to do myself. Some of the seals are clearly split or have moved from a recent visit to the shop. I knew it could happen and was a patch up job, but how easy is it for me to do myself and where would i get the mastic from (is it specialised for the motor home or will any plastic external mastic do?

Thanks in advance for any helpful advice