If they’re too lazy to do exterior cosmetic maintenance, they might have also neglected more important things. Beware of excessive caulk on the exterior- it’s the lazy man’s way of trying to stop a leak without pulling and resealing
windows and vents. Silicone-based caulk is a nightmare to remove from
fiberglass, too.
Check inside very carefully for any signs of water infiltration. Pay special attention to any softness in the floor. Look inside cabinets under
windows for stains or mold, as water can run inside the walls and come out at the floor level. Repairing water damage can turn into a BIG project and would be a reason to pass.
If it checks out dry and the price is right, I wouldn’t be afraid to tackle the exterior finish.
Many have had very good results using Zep high gloss floor wax on older, heavily oxidized units. It gives a nice glossy finish on weathered
fiberglass and fills in the pitting to keep dirt out. Buy online or at big box home stores like Home Depot.
Clean really well, using a pressure wand (with care, avoiding
windows and vents) to get dirt out of the pores, Dawn soap to remove any old grease and wax residue, and Barkeeper’s Friend (again with care- it’s a mild abrasive) on black spots and streaks. If there is silicone, a combination of mechanical and chemical clean-up may be required to get good adhesion of the wax. Once it’s clean and dry, wipe on 5-6 thin coats of Zep with a white cloth and let dry. It typically lasts 1-2 years depending on climate and storage. When it starts to yellow and peel, remove using the Zep stripper and reapply.
Once you get it looking nice, inside storage is the best thing you can do to keep it that way. UV is the enemy of fiberglass and waxes. The top will forever be a little rough, but as long as it's clean and protected, it'll be fine, and you don't really see it in ordinary use.
Shiny gelcoat commands top dollar on resale. It’s typically the first thing to go. Mine was four years old, stored outside, and had never seen a coat of wax. I got it for a really good price, so my labor to restore the exterior finish was well compensated.