First, welcome to iRV2. Now to the RV...
You need to determine which axle you have in your truck.
With the Avalanche you can only pull travel trailers and not 5th wheel trailers.
Next you need to see what the GVWR and GCWR are on your truck and axle combination.
Come back with some more information and we'll help out more.
For starters, slide outs add weight. Practically your your Avalanche (1/2 ton truck) you will be limited to a 24 to maybe a 26' long trailer. The bunk beds may not be possible in this size trailer.
A good website to learn all about towing, ratings and how to calculate whet you can tow is Ken Lenger
You need to understand weights and ratings before you start shopping seriously. The FIRST rule of RV shopping is ...NEVER believe the RV sales person.
The SECOND rule of RV shopping is...RV sales people lie.
The sales person is not your friend and he is not there to educate you and keep you from buying too much trailer for your truck. He is going to push you to the largest trailer he can get you into and will tell you that the truck will pull it like it wasn't even behind you.
Take your time and DO NOT let the sales person pressure you into a fast "today only deal". With the current economy, it is a buyers market and there are plenty of RV's available.
Hope to see you back with questions....
On edit:"Your options are nearly endless with your vehicle, as long as you stay within the towing limits for your Avalance...2WD models with Heavy-Duty Trailering Package is 8100 pounds."
The 8100# is a MAXIMUM based on a properly equipped truck, and a base model with only a 150# driver, no cargo, no accessories, and no hitch. Every pound you add over the base model reduces the towing capacity.
SO you need to get the capacities based on your 5.3L engine and the axle you have. The Avalanche is heavy, so you will need to watch the loaded truck and the available payload capacity and hitch weight.