Hi it will come from Motorhome battery Not the Auxiliary It will all depend how old your van is, If it is quite new the radio will switch itself off after about 1/2 hour.
If it's a standard cab radio and the wiring has not been modified by a previous owner, it will almost certainly be running from the engine starter battery.
The "how long" has a huge number of variables, including "how loud" and "how good is the state of the van battery", but you should get tens of hours out of it before the battery would be too low to start the van.
If it worries you, either change the wiring (this can be exceedingly hard to do on recent vans) or get a battery monitor and make sure you don't let the van battery go below - say - 11.5 volts.
Hi angel 34, you can buy a small solar panel that you put on top of the dash board when on site which i think plugs into the cig lighter which charges you m/h starter battery
------------- Life is to short,be happy have fun.
regards
bald eagle