On the battery, look for a true deep cycle battery. Next best is an RV/Marine but only if you cannot get the deep cycle.
As to size, a Group 24 battery will work. A group 27 or larger will work better if you are doing a lot of off the grid camping. I myself have 2 group 27's but I also do an amount of boodocking and with being a power mizer and LED light, I have never run out of power and do the genny recharge every day or so.
On the slide bars. Mine has the transit bars from Sunline. Mine is a 2004 model. In 2005, Sunline changed their approach to this and stopped using them.
I called Lippert who made my slide mechanism and asked them about the transit bars, they stated they can only commit on the slide mechanism and not on the slide construction. They said see your TT manufacture for that. Well Sunline was no longer at that time so the politically correct answer did not help me much.
In my case, I use the transit bars. I know folks who have the 2005's and I have studied their slide a lot. My son recently acquired a 2006 T264SR and I see what they used on that model as well. All 3 of those model years use the same Lippert slide mechanism.
Your 2003 may or may not be a Lippert slide mechanism. You might have the Barker slide mechanism. I do not know if that mechanism will lock like the Lippert rack and pinion. The 2 mechanisms look very different. See this post with pics of the 2 systems.
Here is a post on what my transit bars look like. I install them on the top of the slide., not the sides.
They are basically pieces of 1/2" conduit cut to the correct length and rubber chair feet on the ends. I added flat washers inside the rubber feet to stop the pipe cutting through the feet. I use them as it helps keep the top of the slide from bouncing going down the road on hard bumps. I have no fear the slide motor drive is going to let out, but the top of the slide moving, is not beyond image on some real hard back country road bumps. The 2005's and newer slides are built similar from what I have found in respect to staying closed. I have also seen "some" 2003 slides and they are built like mine as well. If you have an early build 2003 it might be a Barker. If it is a late 2003 it may be the Lippert mechanism. I would confrim the slide mechanism before making a decision. We at least have some history on the Lipperts and not using the transit bars.
Hope this helps
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
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