I've used the blue crystals you can restore by drying in an oven. They work. Sorta...
They absorb the moisture in the air of the camper, but use so much energy to dry them back out, AND I have to do it every week!! They go from a midnight blue to pink in 7 days easy. This is with a brand new Lance 855S with a cover over it too. Next, I tried a dehumidifier. What a waste of money! The crystals worked better. I'd weigh the crystals before and after use to see how much water absorbed. I weighed the water from the dehydrator and the crystals out performed the dehydrator by about 50%.
Next I used a heater. Not an oil filled but still, one that doesn't use a fan. Fans quit working. I wanted something with no moving parts like an oil heater but could be permanently built in. Found a baseboard heater that did the trick. Now it stays dry. How do I know? I put the blue crystals back in with the heater and the crystals stay blue for at least 4 weeks.
The small oil filled heaters that look like a radiator should do the trick on the lowest setting and the thermostat set for 62*.
There's a huge benefit to using a heater as well; water in my lines don't freeze up. It's much easier to toss in a heater than it is to purge the lines since I use my camper all winter long; at least twice a month.