a) big trash bag or dry bag to put the pack in at night. Close tightly and store as far as possible away from beds or even outside dormitory (especially if you plan to walk early, the trash bag will be noisy).
b) permethrine spray or "Clako anti punaise" (french all-natural product you can buy in St. Jean) to spray on your pack and sleeping bag. The Clako doesn't work long term so needs to be re-applied every few days (apply outside to not affect other pilgrims with the smell). Please know that permethrin is deadly to cats, so don't use that if you have cats at home or stay somewhere with cats. These products don't repel them but at least the bugs have a good chance to die if they are in contact with it for a longer time, so you're less likely to carry them with you.
c) liner that is big enough to crawl into completely, including head. A permethrin- treated one is a good idea ("insect shield"). Bed bugs can't walk through the fabric so if you cover most of your body you'll have less bites even if there is an infestation. Doesn't have to be silk, but it is lightweight and nice on the skin
d) dry bag for sleeping gear. Put all your sleeping gear that touches the bed in an extra dry bag. That way, if you notice bites during the day, you only have to treat that and not all of your gear once you arrive at the next stop.
e) Medication that helps if bites occur - citerizine, anti-inflammatory creams, ect. (also available in spanish pharmacies, some only with a doctor's prescription).
I've had bites / seen people with bites / seen signs of current and former bed bug infestations in albergues on all my Caminos in France and Spain so far. But I react badly to them so I'm very careful and maybe check more thouroughly than others.
Look out for signs like the one on the picture. It means there has been an infestation at least at some point (bed bug poop). Usually you'll find it in mattress seams, in crevices of the bed frame, sometimes behind pictures on the wall ect. Talk to the hospitaleros then, the problem might already have been resolved and only the poop is left but no bed bugs
Wish you a bed bug free buen Camino!