Welcome to the club. As far as a manual for your Sunline there isn't one. Sunline manual was made up of different appliance manufacturer manuals.
If you go to Sunline owners club at the top of page click on file tab then brochures you can find your trailer model specifics. Like length, dry weight ect.
Amazon has replacement hot water tank kits also complete hot water tanks. I changed out just my water tank on another trailer a few years back, not hard. I suggest when installing a water tank put a drip pan under it so any leaks won't rot floor.
Jim and DW
2001 T-2553 Sunline Solaris
2006 GMC Sierra Duramax 2500HD 4X4
Firestone Transforce AT tires
Reese Dual Cam Sway Control