New Roof Leaking
Can't answer specifically without seeing it, but you obviously have a flashing problem. The only way to flash this successfully is to use a base flashing tall enough to go from the new roof to the top of the mobile home roof, then come down from the mobile home roof over the base flashing was a counter flashing, which should be lapped at least five or six inches onto the mobile roof and completely bedded into it. I would also have this counter flashing broken with a four inch wide kicker at the bottom that matched the slope of the addition roof and laid right on it when installed, to divert water away from the connection. Any type of flashing that merely turns vertically up the side of the mobile home and counts on some type of caulking to waterproof the top of it is doomed for failure.