No, because that's not a permanent residence. Each state has their own residency requirements, so you'd have to comply with the residency requirements of whatever state you want to be a voting resident of. This is one of the issues called out in the current voting fraud scheme: people illegally voting with an RV park / campground as their address.
PO Box addresses are not allowed to be used for registering to vote. You have to use the physical address at which you reside.
What do the people who sell it all and live retirement in a huge RV year round? Can those people not legally vote?
No, because that's not a permanent residence. Each state has their own residency requirements, so you'd have to comply with the residency requirements of whatever state you want to be a voting resident of. This is one of the issues called out in the current voting fraud scheme: people illegally voting with an RV park / campground as their address.