[MrE]

My 86 was a direct drive alternator.

[kyleb]

From what I remember, to access my engine from under the bed, I would have to disassemble some cabinetry - It is definitely NOT made like my Dad's Thor DP - that hinges up easily.

Mine has a heart inverter & a engine coolant heater under the bed along with a couple of drawers.

[Floridian]

Super amazing stuff here. My wife told me “You should look for that belt in the toolbox” I think I don’t want her to be right! I’ll take a look at it tomorrow and also record a video of the location of the alternator and what I have under the bed (inverter, drawers and heater that works with the engine coolant)

[Tf175]

The DD parts manual shows eight belt configurations , from 28" to 46" in length . Width 3/8 and 1/2 .  Engine serial number and type are needed to find correct P/N . 
Hopefully you have a replacement belt in your spare parts bin . 
When bed is removed on my '86 the engine is accessible,  alternator is easy to get to . A section of the bed base has been cut out at some point to allow engine access . This panel is held down with screws , and is layered , hard insulation sandwiched between top piece and bottom plywood which has foil backed foam insulation . I avoid removing all this if possible !
My air cleaner is on the passenger side , so  I can get to the alternator belts from driver's side . 
Issues I had with the direct drive alternator that led to
my belt conversion is another story for another day. 
Good luck! 
Chris