Look at:
Canvas Pull a bit on seams especially where they meet the corners of the
bunks make sure the stiching is in good shape. Make sure the canvas is
in great shape. Dry rot in the canvas it won't last very long at all.
Look for areas around the corners inside on the ceiling and at any point
where water may leak. Check the outside corners see if they have been
sealed recently.
Plug it in check the electrical hook it up to a truck car and check the
inverter make sure it will work off of 12 volt if needed.
One thing most people forget about is the lights... check the light
fixtures take the lenses off you will be able to tell if the roof leaks
very quickly. Check the lift cables for rust and wear.
Use good common sense and you should be ok... I've rebuilt a number of
the old Colemans and have had a ball doing it. I learn something new
each time. Any questions send me a e-mail.

Looking at a small Coleman that I'm guessing is from the mid to late 80s.
It's similar to this one:
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&category=50064&ite
m=4641261367
Appears to be in good shape. Anything in particular that I should watch
for? Looking to make the jump from tent camping with the family. Plannning
on towing with V6 Amigo and not going cross country (4-6 hours out).

Try to poke your fingernail into the ceiling (without damaging the vinyl)
to find hidden soft spots. The construction of the roof can lead to hidden
water damage, and it's hard to repair. Be especially careful to inspect at
corners and along the joints. If it had A/C, taking the inner shroud off to
inspect the hole cut through the ceiling would be a good idea. It's a
common point of water intrusion.

Check all the electrical circuits. Turn everything on/off. Check the
running lights, stop/blinkers, etc. If it doesn't have a live battery,
borrow one to hook it up. That way you can test the 12v circuits off shore
power. Check it on shore power, as well. Turn on the furnace to make sure
that works. If the thermostat doesn't, that's probably an easy repair.
Fixing the furnace itself would be more expensive.

Look underneath at the lift mechanism pulleys to make sure they're intact.
If the owner had it set up before you get there, ask to take it down and
set it up again if you're serious about buying. You have to see that the
basic mechanism works.

On a trailer of this age, the pluming and other hardware is relatively
simple. If you're reasonably handy at fixing things, you can do most of it
yourself.