I do not know the what Sunline used back then, however in standard tubing sizes in the range you are looking at:
1/4" OD (Most likely too small)
3/8" OD ("might" be)
1/2" OD ( this you already know)
If you have a pair of calipers you can measure the OD or if you can get to an end of it, use a tape measiure across the end of it.
Hope this helps
PS Home Depot and Lowes does have potable water 3/8" BUT make sure it is hot water rated.
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
Google Custom Search For Sunline Owners Club