Im no expert but i have a Savannah monitor who i noticed was being sluggish as well. I decreased the food intake. I was feeding my sav two mice every other day, way too much. I noticed he was not moving and exploring at all. If your sav looks like a football in the middle he/she is eating too much. Also check your temperature some say basking spots of 100-130 but i found my sav doesn't like it that hot he likes it around 90-100 degrees on the hot side and 70-80 on the cool side. If lizards are too cold they wont move. Like i said im not an expert i just had my fair share of problems with my sav and i use my common sense to help, and it generally works. Also remember monitors are fairly new to the pet industry don't always trust the advice of some so called experts, your sav will tell you what he/she likes when you observe them, raise the temp feed enough not too much If that doesn't work take it to a vet.
As for monitors I've heard both stories about uvb but honestly my opinion is that if they get a proper diet, uvb is not necessary since they will get all their nutrients from it, unlike other omnivores and herbivores. However, if the diet you are supplying is not enough (you'd have to explain your diet regimen) then you might need to supply uvb. One symptom of the beginning stages of mbd or insufficient uvb is sluggishness. Most people I know with monitors don't need uvb but one of my friend whose monitor was heading that way, started using the uvb that I use for my beardies and other of my diurnals (zilla desert 50). I really like those, and his monitor started doing well. But obviously there are other good bulbs like mega rays and some zoo meds you can get.