I just had to add my 2 cents to this thread.
My son recently bought a 30-year-old travel trailer in remarkably good condition. The tires were cracked, but held air but the guy who went to look at it with him refused to tow it because of the tires. So the seller delivered it because my son did not yet have a suitable tow vehicle.
I said the tires had cracks in the sidewalls. They were also L78-15's, the same as specified on the nameplate and I'm guessing they were the original tires. Of course they had to be replaced. He took the wheels in to a tire shop for replacement tires and when the old ones were removed, they looked a lot worse on the inside. I know that with trailer tires people won't always go strictly by the date code on the sidewall and replace them after 10 years because the tread is hardly worn. Hell, some dealers will even try to sell you "expired" tires to make a buck if you don't understand date codes. However, from our experience, a tire does not have to look bad on the outside to be bad on the inside.
Oh, and about grease in the bearings, what a previous poster said about cars is true. If the grease is clean and not contaminated or black or dried out, it's perfectly good. If the seals aren't worn out, letting out grease and letting in water, they're also okay. If you've been a shade tree mechanic for 60 years like I have, you can make decisions like that. If not, replace the seals and even bearings if you have the time and money.