First, you can do the "axle flip", or more correctly, the over/under conversion. Here's a link to the instructions for the Dexter version, right here on SOC in the files area:
Sunline Coach Owner's Club - Sunline Owner's Files - Dexter Axle Flip Kit
The over/under should give you 3"-4" of immediate lift. It does this by relocating the axle from above the spring pack to below it. It is an easy mod, and can be done by most DIY'ers. There is some tack welding required to finish the install. If you can't do the welding yourself, it would be OK to gently haul the trailer to a welding shop after the install.
Second, once you've done the "axle flip", it might be possible to go to a larger diameter wheel and tire. You'll have to be the judge of that as some careful measurements will be needed to maintain adequate clearances. Keep in mind that it is highly unlikely that a wider tire will fit when looking at larger diameter tires.
Third, it is also possible to increase the lift of the suspension beyond what the "flip" kit offers. There are several ways to do this: One is to add spacers between the axle and the spring pack. It is also possible to install a spacer between the spring hangers and the frame of the trailer. Either would require the services of someone who really knows trailer frames, suspensions, and axles. My recommendation would be to avoid RV dealerships and RV repair shops as they tend not to think outside the box and their shop people are not good with one-off modifications.
Instead, find a shop that deals in utility trailers. They generally do better suspension, axle, and brake work, and at less expensive shop rates. All the components used in RV undercarriages are exactly the same as those used in utility trailers and even boat trailers.
By lifting the trailer the 6" or so that you need, you will also create another challenge in the added height of entry and exit. But given the work you have already put into the trailer, that may be a pretty minor issue.
Good luck with this, and do let us all know how it works out for you.