Yes, it's possible for us to install our loo trailers inside marquees and this is something we frequently do over the colder months of the year. A covered loo trailer is a plush addition which will wow your guests, particularly as they don't need to get wet, cold or muddy by venturing outside.
Usually marquees are installed 2-3 or even more days before events take place. Depending on access, we would need to either deliver and position the loo so the marquee can be built around it, or if the marquee size allows (eave height in particular) we can manoeuvre into place afterwards, say the Friday.
Worst case scenario, we would need to put the trailer in place a few days prior and then return to get the system working and add the magic blue fluid the day before the event. Depending on the location this might attract a small extra charge.
When planning your marquee be generous with space: 6m x 6m minimum but ideally 6m x 9m so you can set up a cloakroom and 'makeup adjusting area' alongside the loos for the ultimate splendid touch!