I just ran a 30 amp circuit to an RV box. We ran 6 gauge NM-B 75 ft through the attic to a plastic box in the attic and connected to thhn/thwn and ran through 1 inch pvc 18 inches under ground to the box 75 ft away. Total of 138 feet plus rv cord. Voltage is good and always above 117 v. I dont run the AC and Micro together but it will do it without popping the breaker.
The THHN at Home Depot is dual rated as THWN , it is needed for wet locations. Even though it is in electrical pvc it is still considered damp wet.
By code pvc has a percentage of "fill" rating. They dont want the pvc stuffed full because of temp ratings etc. So one thing you need to do is talk to an electrician etc to be sure your fill isn't over with whatever you end up with.
You also cant run romex in pvc
Right now it's at 117 volts/19 amps. The MN-B that we ran through the attic is also rated at 90 c or about 197 degrees. Three strands of 6 thwn had a lot of room left in the pvc but it is what the charts suggested. This panel is on the exterior of the house so I ran a 2 inch pvc stub out the top of the panel into an LB and then horizontal into the attic. That 2 in pvc has one 10 g, one 14 g, three 12g and one 6 g romex and it's probably about 1/2 full give or take. So I'd say that you can get X amount of conductors through X size of pvc but it may not be code. They want extra room in the pvc which you most likely have.
You cant run romex in PVC.