Congrats on your new 5th wheel. You will soon see that choosing a Big Country is the right choice. Now...the PDI is always a hot topic on the discussion blogs on here. I'm sure there will be many people that will have many things to tell you to look at. But if it were me, and I had a very poor experience with the PDI when I bought mine in January...here is my 2 cents about what to look for.
1. Plumbing. You will want to make sure water is hooked up to your RV and when you start your PDI and get to the plumbing section, turn on all faucets, and check the drains one at a time to make sure they are all tight and not leaking. Next, and I highly suggest you do this, have them take the all out in the basement to gain access to the back of all the water valves in the UDC. This is the heart of the water system and you are going to want to check every clamp and elbow to make sure there are no cracked fittings. I have replaced 5 out of 6 fittings because my unit must have frozen in the winter from a poor winterization and it presented numerous leaks that are very frustrating. SO ask them to remove the panel. It's only 6 screws and very easy to put back.
2. Storage tanks. The gray 1 and 2 tanks. Make sure you fill them completly and then drain them completely. Make sure the sensors are working and indicate correctly. You will want to make sure you fill the freshwater tank with enough water to pump to all the facuets and shower as well. Make sure the pump keeps up with the demand and has no leaks too. Again, having the panel removed in the basement will help you with that.
3. Electrical, make sure you turn on every light and try to bring a portable appliance or lamp to check every plug. Make sure you have power going everywhere it's supposed to.
4. TV and Stereo. My outside speakers are blown. I have new ones on the way from Jensen. But somewhere in the travels of my RV, someone turned the stereo on very loud and did not know the outside speakers were on and blew them. So check this and make sure it all sounds good to you. I use DirecTV and the satellite wiring to the bedroom had a bad connector on the wire. It was a quick fix at the dealer and I know you will probably not be able to check that until you are home, but it's just something to keep in the back of your head.
5. Make sure all of your tanks empty in a reasonable amount of time. I have two back water tanks on my 3490RB and the aft tank gate valve had some trash in it and it allowed fluid to fill the outlet when the cap was on. I have cleaned it and it's working fine now, but you don't want to find that when you are camping. Just make sure that when the tanks are full of water at the dealer, that you can take the outlet caps off and nothing comes out.
6. You are getting a brand new unit and I'm sure that the sealant is going to be fine, but keep a watchful eye on the edges of everything. Look for gaps in sealant and make sure they are all in tact. Nothing is worse that water getting into the RV from that.
7. With the RV perfectly level at the dealer, I suggest you have them install a level on the pin box. Check the door/s and make sure they open and close with no hanging. You won't regret it later when you are trying to level the unit and you don't know if you got it or not. There have been many discussion threads about leveling the unit and since it's your first unit, have the dealer level it up and install the master level on the pin box and then you will always know for the future. It's well worth the small expense now to prevent aggrevation later.
8. Last but not least, tires. Make sure they show you the pressures on the tires before you drive down the road. Nothing is worse than a blown tire. You won't regret it. Make sure your truck is aired up and ready as well.
Well, I congratulate you on your decision to buy Heartland and Big Country. We are very happy with ours, and look forward to many many years of RVing in it.