How many miles does it have? Regardless, it needs a complete once-over by a qualified truck mechanic. Fluids (including fuel), belts, hoses, metal parts do not like sitting still not being used... they break down quicker that way. If replacement is needed on those, better to do it now then find yourself on the roadside with a failure of some sort. That'll happen eventually anyway if you RV long enough!
Has it been stored indoors or out? Roof condition (especially the seams) should be checked, along with any sign of past/present water leaking from roof or windows. Look for any water stains or odd odors in the rig, on the ceiling, walls, floor, inside cabinets, and anywhere else you can think of. Flip every switch and test every possible function inside the rig. Fill up the plumbing system with water, run every faucet/shower/toilet and check for leaks.
Unless you are getting a killer deal on this rig, I'd personally rather buy one with higher miles that has been regularly used and maintained. Too many unknowns with an RV that has been forgotten about and possibly neglected for 7 years.
As Scotty suggests, I'd do a comprehensive chassis service, at a reliable truck shop. I'd also replace all the tires, as they are obviously far too old and risky. Engine start batteries too, if any doubt (you said it started ok).
Then the "house" stuff:
Inspect the roof and touch up lap sealant (seems, openings, etc) as needed.
Inspect around windows, ceiling, floor, etc for signs of leaks
There's a lot of info on the first generation Senecas over at jaycoowners.com forums.
The GM medium-duty C5500/Kodiak is a good chassis. The diesel was a 6.6L Duramax. LLY for 2006 and beyond. Lots and lots of info on the C5500 chassis specifically, and on the Duramax diesel in the medium duty chassis over at dieselplace.com, including the Allison trans. The trans is stupid-simple to service and even has a spin-on fluid filter.
I've got an '06 Seneca on a C5500 and have been through it inside and out.
As with any vehicle that sits, your main concerns are seals and corrosion. The exterior from weather, if it sat outside, and the mechanicals from sitting full of stagnant fluids. If you don't know what you're looking for, find an RV inspector who does.
I work on mine with basic tools, there is plenty of room on these things, very easy to maintain. Mechanical parts are mostly parts-bin GM stuff and it's all widely available because a lot of it was used on other platforms and in GM's light-duty line as well. Ours was low mileage and 15 years old when we bought it, had arguably spent some time sitting. It's been a great rig for us.
The key is, with anything that old, you'll want to start fresh with filters, fluids, tires and everything consumable. If you can't do that work yourself, you'll be paying. But it's important stuff no matter what the rig. Happy to share any experiences with my Seneca if you have questions.
I've shared this before, but when I bought my Bounder, as soon as I got it home from Houston I spent $6000.00 on it before I used it at all. New tires, ALL fluids and filters at Cat dealer, complete transmission service, front end alignment at Freightliner, and genny serviced at Cummins/Onan.
And it's been a GREAT motorhome. I've only had a few minor squawks that I've repaired myself.