Thanks for the quick comments everyone!
I love the method you used laughingindian! We are planning to be boondocking
the majority of the time so installing a full holding tank might be the way to go. We are super low maintenance so we don't need the craziest amenities. How did you insert the tank if you don't mind me asking?
The only issue with this method is that we are hoping to still utilize the fiberglass instert to maintain that whole "original" feel by cutting to fit.
I love the idea of being higher too, so in a best case scenario we could have the fiberglass insert raised up with a toilet tank underneath. I thought I had found some forums with that but cannot for the life of me find them anywhere!
Thanks again for the help everyone!