The typical rule of thumb is 12lbs of grain for a 5 gallon MT. This rule has been broken on multiple occasions by yours truly, but 15lbs is about the max and this puts you at a grist/liquor ratio of .9, which is bordering on problematically thick. So the simple answer here, is you will need to explore other options. If you are fly sparging, I would recommend a larger tun. If batch, then loading your tun twice or using a second tun would suffice.
When in doubt, check out the "can i mash it" calculator on my club's page here: