Welcome to the forum. Sunline logo is in the strips and you said you found 15 1/2 SB in the cabinet, thus I believe you have a 1982.
1981 Sunline 15 1/2 SB had Sunline logo above the strips and in 1983 the Sunline 15 1/2 SB was called the T-1550.
You can see some of this by going to the "FILES" tab at top of this page and then clicking on the "brochure" tabs and then "download". You can look at the various years.
Your unit may very well be a 1882 model year that was built in 1981.
Sunline Fan is the expert on these things though.
TT:1983 Sunline T-1550
TT:1996 Sunline T-2053
TV:2005 Toyota Tundra Double Cab 2UZ-FE i-Force 4.7 L DOHC (MFI) V8 4WD SR5 Automatic
P3 break control
"I know a lot about nothing and nothing about a lot"