I have replaced a roof on my 73 TT in 1986 due to the same issues you have. Its a job, not difficult to a handyman with basic carperntry tools.
MY job was a 1973 20' TT with a n aluminum roof made up of 4' panels fold seamed togrther that leaked over 2x2 lumber tapered to 3/4" at the walls.
I removed all the roof to the inside cieling panels and started over. I framed the roof with 2x4 lumber with 1 1/2 arch at the eave to 3 1/2" at the apex which allowed greater insulation and rain snow shed.
Being that campers are built from the inside out, the new luan ply ceiling panels must be placed first, then the trusses, framing, wiring insulation , lastly roof which I did with a 1 piece aluminum sheet roof.
Bought that unit in '78 for $5k, full timed in it for 2 years sold it in '89 for $ 4500 better than new.
Asagging roof is due to thin joists
For me a rebuild paid off very well.