Paulmlac, should be able to find the larger threaded plug/access hole piece at the boat or marine place as mentioned before. RV dealer may have one. Seen them used for plumbing access in rv showers too. I could have swore I've seen them at Academy before, or maybe something similar. Cut away. Easy fix.
John and Rebecca Dickson
Emma-7 / Little John-5 / Iva-1
Full Timing Again, Rev B
2013 Ford F-350 Lariat CC LB PSD
2015 SOB TT - With OC's Awning Poles (#8)
At least in Heaven, RVs will be perfect, and I won't have to keep fixing them.